What is the justification for the use of discipline/ punishment in schools?
Discipline is necessary in any organisation. The problem is, how to enforce that discipline. The problem becomes even more acute when dealing with children; but there is no avoiding the fact that discipline is a vital part of a young person's education. A young person requires guidance until s/he reaches an age where they can make judgements and weigh-up situations for themselves. Education is not just about academic subjects, it also includes, what was called by my generation, learning how to become a good citizen. Part of this education was learning good manners, learning to respect others, and a general idea of what was considered good and what was considered bad or evil. A major factor, which people often overlook, was self respect; it is difficult to respect others if we have no respect for ourselves. Underpinning all this was the firm imposition of discipline.
Of course, discipline of children was always considered to be the main responsibility of parents, the home was where good manners were expected to be taught, where parents were expected to set a good example, however, schools did take it upon themselves not only to back-up parents but to take a lead. A good hiding for a misbehaving pupil at school was acceptable to all, and was expected from the most famous public school in the land down to the most basic Council School. It was not unusual for a young person to be punished in school and on arriving home to discover that their parents knew of this would be subject to either another good hiding, or being sent straight to bed without their evening meal.
For those of us old enough to make the comparison there is, without doubt a massive paradigm shift in received knowledge regarding morality and ethics. This shift from Victorian times, through Edwardian Times, through the twenties and on into the sixties was hardly noticeable. the changes in the structure of society the began to accelerate at such a pace that the culture shock had a devastating effect on relationships between parents and their, particularly, teen-age offspring. Still living in the past paradigm and reluctant to accept the changes taking place, the earlier generation were left gasping like goldfish in a bowl that had been deprived of nearly all their life-giving water. The gap between the previous and the developing generation opened rapidly, the relationship between young people and adults changed as a different concept of discipline replaced the old concept. One of the major changes was the different attitude towards discipline in schools, corporal punishment was eventually banned, and a rather strange shift of emphasis from discipline of the young person to protection of the young person took place. Physical contact between teacher and pupil took on a sinister meaning: teachers began to feel confused and helpless, their world was turned upside down almost overnight. I distinctly remember a friend of mine almost losing his job because he placed his hand on the shoulder of a girl as she refused to step into line in the play-ground, blatantly refusing to obey the teacher she was quite prepared to hold up the entire school. She told my friend to take his hand off her shoulder because that was a form of assault and he would be reported. He appeared before the Head and the girl's parents the following day, was completely humiliated and warned about his future conduct. Bizarre events began to be reported in the media, like the one where the teachers were besieged in a high school by angry pupils who had taken offence at the attempted imposition of new instructions. Damage was done to staff cars, tyres let down, etc. before the police arrived to disperse the pupils.
The relaxing of discipline in schools initially brought about a lowering of academic standards, a high percentage of pupils leaving primary schools were poor at reading writing, Whether co-incidence or not, there was a continuing rise in truancy, vandalism, attacks on the elderly by young people, drug addiction in the young, thieving and general criminality in all areas. Along with this came a marked increase in general bad behaviour, decline of good manners, and lack of consideration and respect for others.
We must not overlook the fact that other changes had taken place in society which coincided with school discipline problems, the main one being changes in family life. The increasing divorce rate, a lack of interest in the sanctity of marriage, children having two homes with father and partner in one and mother and partner in the other, and so on. In addition to all this came a rapid decline in church going, and the almost total decline of Sunday Schools, both have had an effect on the discipline of young people. Ironically, these changes, which to some are catastrophic events, throws the responsibility for discipline back on the schools.
It is, of course, not the case that all young people fall into the categories indicated, I believe we have all met or know some wonderful young people who are a credit to their school and to society. Unfortunately, the daily exposure of the criminal behaviour of young people shows a continued increase. Even as I write this I am informed by the media that 17,000 juveniles stole vehicles last year. Add the thousands of young drug addicts, muggers and thieves over the same period, and we get the feeling that we are not just on the verge of anarchy, but that it has already arrived. It seems that not only do we need real discipline back in schools, but we also require a tightening of the lenient laws governing this country. When the do-gooders that brought all this about realise that there is a difference between assaults on children and regulated punishment for their own good, and for the good of society, we shall happen see a change back to normality, but don't hold your breath, and don't forget that the bias has swung so far towards the welfare of young people that parents can be taken to court by their own offspring who have decided that discipline is not for them.
I am obsessed with this mind issue. Last answer was highly satisfactory. I began to read the references you sent me. Thanks a lot. In this one I want to bore you with some novice theories about mind, if you allow me.
I believe that mind works pretty much a Hierarchical Petri Net. In each level there is one statement, that can be modeled with places and transitions.
Let "Is Eric Good?" be a question asked to me. Let this question in its context be defined with two places and one transition, when I am subjected to this question.
In lower levels, I have multiple small nets in various levels and details connected to the concept "Eric", as well as "Goodness".
"Eric hit me", "Eric's sister is Jane", "Eric went to college" etc. "Elephants are good", "Bad is not Good", "Getting beaten is bad", etc.
These chunks are further linked to others like "Jane is a girl", "Jane is good".
I believe that man keeps his experiences in chunks like the ones above, very tiny nets with one or two places and transitions. However when asked a question, or while making an exercise man gets into these small chunks, decomposes each of these transitions and places, forms up new nets during run time, and aggregates them to come at a conclusion which is also a tiny net.
Turning back to my example, I conclude that "Eric is Bad". When asked the question, I simply run through the nets in the lower level, associate "Eric hit me" as "Something Bad" and conclude that "Eric should also be bad".
Now what I do is not just finding a "path" in the mathematical sense. If it was so I could easily come up with "Jane is Good" as an answer to the question. The process involves "path finding" but is much more complex.
Anyways, I was thinking that this was a good beginning for a modeling effort about how mind works and wanted to share it with you. I need to read more about this type of research.
Could you recommend me a couple of books/articles: 1) that looks at the issue through hierarchical nets and graph theory, preferably Petri Nets. 2) that contains my theory about runtime graph formation. 3) If people tried to explain this thing through graph theory and could not succeed please let me know about that as well. I do not want to waste a life on this :)
I assume you know about this site, then: http://www.daimi.au.dk/PetriNets/.
But here's one general point. There are many, many ways to approach and to model the mind. Various types of networks are one, of which Petri and graph theory are two categories. It may be that they are formally identical. I will give you some other references below; you can find all the Petri stuff you want at the above site. The danger, if you have not yet done much reading, is to take one of those as the way, and spend enormous amounts of time and energy at it. This is the danger of too narrow an education in any field. IF your goal is to take the Petri Net and elaborate it, and see what can be done with it, then, fine, go with that particular approach. IF your goal is to model or to think about the mind, you will be taking one out of dozens of approaches and effectively saying that it is the correct one. But it isn't, and here's why (and I'm sure many Petri people and others will disagree with this): the brain is an analog system, not a digital system. A computer is a digital system. Can a digital system "model" an analog one? Yes, certainly. What does "model" mean? Now, there's the question. I will not go into that here; suffice it to say that there is ongoing debate on that point. Second, can a digital system duplicate, functionally (obviously it could only be functionally), an analog system? This is another question, not the same as the last. If you want to create a mind in a computer (a digital computer), you've got major problems, I think... indeed, I don't think it's possible. But you can model one, up to a point. There's an important distinction here that many have missed.
So, what approaches should one take to a) model and b) duplicate mind? But you see that these are two different questions. Your approach above will not duplicate mind. But it might model it to a certain extent.
For history (and good background) in modeling and networks:
Ashby, W. R. Design for a Brain: The Origin of Adaptive Behaviour. London: Chapman and Hall Ltd., 1960.
Rosenblatt, F. Principles of Neurodynamics: Perceptrons and the Theory of Brain Mechanisms. Washington, D.C.: Spartan Books, 1962.
Minsky, M., and S. Papert. Perceptrons: An Introduction to Computational Geometry. Cambridge, MA: The MIT Press, 1969.
McCulloch, W. S. Embodiments of Mind. Cambridge, MA: The MIT Press, 1970.
McClelland, J.L. Parallel Distributed Processing: Explorations in the Microstructure of Cognition: Psychological and Biological Models. Edited by J.A. Feldman, P.J. Hayes and D.E. Rumelhart. Vol. 2, Computational Models of Cognition and Perception. Cambridge, MA: The MIT Press, 1986.
Rumelhart, D.E. Parallel Distributed Processing: Explorations in the Microstructure of Cognition: Foundations. Edited by J.A. Feldman, P.J. Hayes and D.E. Rumelhart. Vol. 1, Computational Models of Cognition and Perception. Cambridge, MA: The MIT Press, 1986.
Dreyfus, H. L. What Computers Can't Do. Cambridge, MA: The MIT Press, 1972.
Gurwitsch, A. The Field of Consciousness. Edited by A. van Kaam, Duquesne Studies: Psychological Series. Pittsburgh, PA: Duquesne University Press, 1964.
Husserl, E. The Idea of Phenomenology. Translated by W. P. Alston and G. Nakhnikian. Fourth ed. The Hague, Netherlands: Marinus Nijhoff, 1970.
Merleau-Ponty, M. Phenomenology of Perception. Edited by Ted Honderich. 1st ed, International Library of Philosophy and Scientific Method. New York, NY: Routledge & Kegan Paul, 1970.
Relatively early cognitive & modeling refs:
Allport, A. "Visual Attention." In Foundations of Cognitive Science, edited by M.I. Posner, 631-682. Cambridge, MA: The MIT Press, 1989.
Deese, J. The Structure of Associations in Language and Thought. Baltimore, MD: The Johns Hopkins Press, 1965.
Fodor, J. A., and Z. W. Pylyshyn. "Connectionism and Cognitive Architecture: A Critical Analysis." Cognition 28 (1988): 3-72.
Gardner, H. The Mind's New Science. New York, NY: BasicBooks, 1985.
Gregory, R. "Perceptions as Hypotheses." Philosophical Transactions of the Royal Society of London Series B, Biological Sciences 290 (1980): 181-197.
Grossberg, S. "How Does a Brain Build a Cognitive Code?" Psychological Review 87 (1980): 1-51.
Johnson, M. The Body in the Mind. Chicago, IL: University of Chicago Press, 1987.
Koffka, K. Principles of Gestalt Psychology. 2nd ed. New York, NY: Harcourt, Brace & World, Inc., 1963.
Mead, C. Analog Vlsi and Neural Systems. New York, NY: Addison Wesley Longman, Inc., 1988.
Neisser, U. Cognitive Psychology The Century Psychology Series. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1967.
Pollack, J.B. "Recursive Auto-Associative Memory: Devising Compositional Distributed Representations." Proceedings of the Tenth Annual Conference of the Cognitive Science Society, Montreal. Cognitive Science Society 1988.
Posner, M.I., and S.J. Boies. "Components of Attention." Psychological Review 78, no. 5 (1972): 391-408.
Rosch, E., C.B. Mervis, W.D. Gray, D.M. Johnson, and P. Boyes-Braem. "Basic Objects in Natural Categories." Cognitive Psychology 8, no. 3 (1976): 382-439.
Shallice, T. "Information-Processing Models of Consciousness: Possibilities and Problems." In Consciousness in Contemporary Science, edited by A.J. Marcel and E. Bisiach. New York, NY: Clarendon Press, 1988.
Shepard, R. "Attention and the Metric Structure of the Stimulus Space." Journal of Mathematical Psychology 1 (1964): 54-87.
Shiffrin, R. M., and W. Schneider. "Automatic and Controlled Processing Revisited." Psychological Review 91, no. 2 (1984): 269-276.
Treisman, A.M., and G. Gelade. "A Feature-Integration Theory of Attention." Cognitive Psychology 12 (1980): 97-136.
More modern refs:
Baars, Bernard J. In the Theater of Consciousness: The Workspace of the Mind. 1st ed. New York, NY: Oxford University Press, 1997.
Chang, F. "Symbolically Speaking: A Connectionist Model of Sentence Production." Cognitive Science 26 (2002): 609-651.
Craik, F.I.M. "Levels of Processing: Past, Present . . . And Future?" Memory 10, no. 5/6 (2002): 305-318.
Demetriou, A., G. Spanoudis, C. Christou, and M. Platsidou. "Modeling the Stroop Phenomenon: Processes, Processing Flow, and Development." Cognitive Development 16 (2002): 987-1005.
Dipert, R.R. "The Mathematical Structure of the World: The World as Graph." The Journal of Philosophy 94, no. 7 (1997): 329-358.
Fauconnier, G., and M. Turner. "Conceptual Integration Networks." Cognitive Science 22, no. 2 (1998): 133-187.
???. The Way We Think: Conceptual Blending and the Mind's Hidden Complexities. New York, NY: Basic Books, 2002.
Gernsbacher, M. A. Language Comprehension as Structure Building. Hillsdale, NJ: Lawrence Erlbaum Associates, 1990.
Gopnik, A., and A.N. Meltzoff. Words, Thoughts, and Theories. Edited by L. Gleitman, S. Carey, E. Newport and E. Spekle, Learning, Development, and Conceptual Change. Cambridge, MA: The MIT Press, 1998.
Grossberg, S., E. Mingolla, and W.D. Ross. "Visual Brain and Visual Perception: How Does the Cortex Do Perceptual Grouping?" Trends in Neurosciences 20, no. 3 (1997): 106-111.
Halford, G.S., W.H. Wilson, and S. Phillips. "Processing Capacity Defined by Relational Complexity: Implications for Comparative, Developmental, and Cognitive Psychology." Behavioral and Brain Sciences 21 (1998): 803-865.
Harnad, S. "The Symbol Grounding Problem." Physica D 42 (1990): 335-346.
Kahana, M.J. "Associative Symmetry and Memory Theory." Memory & Cognition 30, no. 6 (2002): 823-840.
Libet, B. "The Timing of Mental Events: Libet's Experimental Findings and Their Implications." Consciousness and Cognition 11 (2002): 291-299.
Maddox, W.T., F.G. Ashby, and E.M. Waldron. "Multiple Attention Systems in Perceptual Categorization." Memory & Cognition 30, no. 3 (2002): 325-339.
Reisberg, D. Cognition: Exploring the Science of the Mind. 1st ed. New York, NY: W. W. Norton & Company, Inc., 1997.
Rieke, F., D. Warland, R. de Ruyter van Steveninck, and W. Bialek. Spikes: Exploring the Neural Code. Edited by T. J. Sejnowski and T. A. Poggio. 2nd ed, Computational Neuroscience. Cambridge, MA: The MIT Press, 1997.
Rizzolatti, G., L. Fadiga, V. Gallese, and L. Fogassi. "Premotor Cortex and the Recognition of Motor Actions." Cognitive Brain Research 3 (1996): 131-141.
Sloman, S. A., B.C. Love, and W.K. Ahn. "Feature Centrality and Conceptual Coherence." Cognitive Science 22, no. 2 (1998): 189-228.
Sun, R. Duality of the Mind: A Bottom-up Approach toward Cognition. Mahwah, NJ: Lawrence Erlbaum Associates, Inc., 2002.
Wegner, D. M., and J. A. Bargh. "Control and Automaticity in Social Life." edited by D. Gilbert, S. T. Fiske and G. Landzey. Boston, MA: McGraw-Hill, 1996.
Yaniv, I., D.E. Meyer, and N.S. Davidson. "Dynamic Memory Processes in Retrieving Answers to Questions: Recall Failures, Judgments of Knowing, and Acquisition of Information." Journal of Experimental Psychology: Learning, Memory, & Cognition 21, no. 6 (1996): 1509-1521.
The above is a bit, a slice, a small example, a mere taste, of what is out there. Dip into it before you go much further in your own thinking.
Steven Ravett Brown
Time travel is an extremely interesting subject, but is it really conceptually possible?
My straightforward answer is no, it is not possible, no matter how you bend it. But if I left it there, someone else will say, it is conceivable under such and such circumstances. So I'm going to have to invite you along on a little journey of problems, just two or three of them, but all bristling with way-out complexities. I'll try and make them as easy as possible, because it's worth thinking about these matters, and also because our lives are so much under the influence of science and science fiction today that the average person can hardly make out what to believe. And by golly, time travel is part of the fare! You must have noticed how much it is taken for granted, as if there were no argument about it!
Well now, since we have to start somewhere, let's take a peek at the 'space of all possible things/ events/ ideas'. Somewhere in this space you'll find time travel and no doubt millions of other ideas, thoughts, objects, events and possibilities that have been dreamt about. They are all in this 'space' as potentials, waiting to be realised. Yet the first thing to note about the 'space of all possible things etc.' is this: there is no such space; for even the 'space' itself, the concept of this 'space', is part of the 'space of all possible things etc.'! Hence it is not a real space, not a finite, three-dimensional volume, where things happen. So you understand that I'm talking about a conceptual space, an infinite realm with infinite possibilities that (so to speak) travels along with our finite realm of real things and real possibilities. It is the realm of the 'Maybe'.
The importance of this concept of infinity is not well appreciated, certainly not by time travellers. They tend rather indiscriminately to toss finite and infinite states around as if they were lego blocks. They talk about 'worm holes' and 'black holes' and 'big bang', and of 'string theory' and 'quantum flutters', which are all entangled with infinity. But consider that infinity means, by definition, that you can't count what's in it. So when you ask, how many atoms in the universe, you are immediately defining the universe as finite.
Having got this far, what about time? Well, it's really the same problem all over again. Is the universe in 'time' or not? Is time 'in' the universe or independent? Astronomers want to convince us that time was created with the big bang, but there is a big chink in that logic. For if the spread of time is finite, then of course the universe must be finite. And vice versa. But if the universe is finite, then we've only pushed the problem of infinity out of the way, because we are then supposing another universe which must contain ours; and that universe is probably contained in yet another: Russian doll universes all the way down. In philosophy this is called 'infinite regress'.
We're obviously getting ourselves into a huge mess. Let's narrow down our focus and note down a sort of definition: 'God invented time to prevent everything from happening at once.' This gives us a vital first clue to what's wrong with time travel. On this definition, time is a concept of simultaneity. It means that if two separate objects/ events occur such that third parties observing them agree in their happening at the same instant, these parties then have a means of plotting the events on a graph, marking their lines of approach and departure and assigning values (seconds, hours, days) to all changes in position. This graph is a 'frame of reference', which can now function as a tool for establishing the simultaneity of all events that fall within its scope. Evidently to make this work, a point at rest has to be presupposed, called the 'residual observer', around which the other events revolve.
Now another difficulty comes up. When you have three, four, a thousand, a billion frames of reference, practically all of them unknown to us because of the sheer size of the visible universe, the notion of simultaneity suddenly runs amok; our little graph just can't cope any more and you'll find that a second residual observer becomes necessary, then a third, a fourth...and in an infinite universe...? You guessed it: an infinite number of residual observers. Where does that leave our simple concept of time? Doesn't it mean there are as many times' as residual observers? True again.
So this doesn't get us anywhere. We're attacking the whole problem back to front. To find out 'what time really is', we need to put ourselves in the seat of time itself. We need to ride along with time on a beam of light. So let's now confront this issue with a 'practical' example.
Let's say you've been despatched from Earth to Alpha Centauri. In earth terms that trip is going to take four years at the speed of light; that's not time travel, but it will serve for an opener. When you last looked back, you might have seen your parents standing there, waving goodbye. A couple of days later, you look again and still they're there. Patient people! But when you look again a year later and find they haven't moved, you are suddenly jolted into the realisation that, of course, their image is travelling at the same speed with you. Time is standing still for you in relation to that scene.
Now difficult as it may be, try and draw a sound conclusion from this. These are not your parents, but merely their image. What then, if you could suddenly double back and return? The point is: nothing changes; and when you arrive, to your parents you will only have hovered in the stratosphere for a while and then come back down.
Now clearly this is nonsense. You've been en route for a year! Consequently there is an irresolvable contradiction: you cannot, as a physical body, be in two places at the same moment, but this is what the story entails.
It gets worse when you really start time travelling. Imagine yourself accelerating beyond the speed of light. As you gaze out the porthole, you'll see start seeing things you shouldn't: ice ages, continental drift, the earth aflame like a drop of molten iron etc etc. On our diagram of Earth, Alpha Centauri and yourself, your numbers are running into the negative: you've reversed the time relation between you and planet Home.
Now there is another side to this story. To observers on earth you would first dissolve and then disappear. Conventionally we take this to mean that the speed of light can only be attained by electromagnetic radiation (EMR), accordingly your acceleration has the effect of converting you and your craft into EMR. But this in turn means that, in relation to Earth, you have ceased to exist. You cannot therefore simply double back and hurtle back to Earth. She won't be there when you arrive. On your diagram, where Earth and Alpha Centauri comprise a frame of reference in close simultaneity, you have removed the residual observer, yourself.
But ah! you cry, even if I can't return to Earth, yet this is time travel, isn't it? Can't I now connect with another frame of reference?
Well, I promised you this was going to be complex, mind-boggling and irritating. For while you may conceivably exceed the speed of light in relation to your own system, you cannot exceed it in relation to light itself. Here the equation is EMR = Time. The grain of EMR in the universe is also the grain of time, and the best or simplest way to make sense of this is to reverse the notion of speed. To attain the speed of light means, in this context, for you to become decoupled from any frame of reference whatever because you have become connected to the stream of time/ light itself. But this 'stream' being the grain of time itself, means you are standing still again, only this time in relation to the whole universe. Then the objects of the physical universe, galaxies and nebulas and novas, will be fizzying around you in a bewildering torrent of criss-cross patterns across the entire 'sensurround' horizon. Indeed some or many of these objects may actually 'collide' with you, at the speed of light (!).
One last question: could you not 'decouple' from this unwished-for state and return to a definite existence? Unfortunately the answer, once again, must be 'no'. I keep saying 'you', as though there was a 'you' in this EMR stream. But of course, there's not: you have become a beam of light, pure EMR, which contains not the thinnest thread of information. Once upon a time, in your real life, 'you' were (among many other things) a packet of information; this is now gone, terminally erased. And this is of course the real crux of the matter.
Simultaneity is the coincidence of objects (information) in a frame of reference: and all these frames of reference are finite entities which might all, in principle, be co-ordinated in a network of finite observers. But 'behind' this structure is the structureless grain; picture it like a single dew drop somewhere in the midst of the Sahara desert. And in this structureless space all events occur simultaneously, just as the sand in the desert 'occurs' all at once; but for us, who have a finite perspective on them, these events occur in sequence and under conditions to which the concept of simultaneity can be fitted.
I hope all this makes sense to you! If you wanted to put it into a nutshell, you could say that time travel cannot happen because time is not real: it's not a road or a space or a field where you can identify Point A and Point B in relation to one permanent, unchanging residual observer. It is (as I said) the idea of some things occurring measurably simultaneously. So the crucial component (you might have picked this up when you recognised your parents as only an image) is this: that light waves bearing images are not physical reality. On this discrepancy the whole fancy breaks apart. Time travel, so understood, is mistaking a 'report' for the event itself; and of course a report can long outlast the event which has meanwhile ceased to be.
And this brings us back to the 'space of all possible things', where we started. Here simultaneity is meaningless, because in an infinite space nothing is simultaneous with anything else, there is no frame of reference and no residual observer; and indeed, there is nothing whatever in this 'space', not an atom, not a breath. Just dreams of finitude, of finite possibilities. Dreams of being, for nothing in this 'space', nor the space itself, has being.
It Depends what you mean by 'conceptually possible'. I would say that time-travel is logically possible because there seems to be no contradiction in the concept (which is obviously very different from sayings its physically possible in our world.)
The interesting questions, as far as I can tell is what known as the grandfather problem. Suppose that time-travel is possible. Now, suppose you go back to the time when your grandfather is in his youth and you kill him this would mean that in the future, there will be no you. But then how could you have come back from the future and killed him?
Here I agree with David Lewis. He reckons that time-travel is possible but you can't change anything in the past. This is because he thinks of time as a big line and each point is equally real. Consider time T, when you travel back to point T*. Now, Lewis wants to say that point T* is equally real when you travel back as when you are there at point T*. The only difference is your perception of T*. The answer Lewis gives to the grandfather problem is that you can't kill your grandfather or change anything for that matter, for the reason that you were there already. This sounds weird but if you think about it it makes sense.
Travelling through time is something we all appear to do every day, this morning I was in the past but now I'm in the present which was the future! I assume however what you are talking about is when an individual travels to a time outside of the ordinary scope. There's an interesting article in Le Poidevin & McBeath's book The Philosophy of Time on the subject but I can't remember who wrote it, however here are two key issues.
First if we were to travel back in time it would appear possible that we could change the past, possibly causing a causal loop whereby our actions in the past affect the way we are in the future. Second there is the ontological status of the past and the future.
To deal with the first problem, consider the 'Back to the Future' scenario where the character potentially stops his mum meeting his father and therefore prevents his own existence. If this were to happen however it would not be the case that in the future that he could go back and prevent his own existence. The argument therefore entails that if he can prevent his own existence then he can't prevent his existence. The other apparent way to avoid this problem is to suggest that you can't affect the past when you go back, but this is somewhat strange. The way around this problem is to say that the Time traveller can affect the past however he can't change it. the 'past'' is already a determined system which the time traveller may cause an event in but any event that he causes will have already happened. He is therefore free to affect the past but he cannot change anything that happened in it.
The second issue is whether there is anywhere to travel to. There are two main positions on time which broadly are the tensed view and the tenseless view. Without going into the positions too much the tenseless view of time is that there is nothing ontologically privileged about the 'present' that we perceive, all times are equally real, thus this position is somewhat analogous to the conception most of hold on space where there is nothing special about 'here' rather it is just the place we happen to occupy. If you are a tenseless (b-theorist) theorist then there clearly is a 'place' to go to when you time travel.
The second position that is held is the tensed theory (a-theory) of time whereby there is something privileged about the present, namely it is the only time that is present. Time flows from the future into the present, and the present to the past. One of the main motivations for this position is that it allows us to hold that the future is open and allows for a non-deterministic position of the world. The a-theorist has more work to do than the b-theorist at this point as for the a-theorist three main positions are viable:
a. Only the present exists.
Now depending on which of ac you accept you're potential to travel to those places is affected, clearly if you hold a then time travel is a priori impossible, if b then you can't go to the future.
b. The past and the present exist.
c. The past present and future exist.