This book is a defense of modal realism; the thesis that our world is but one of a plurality of worlds, and that the individuals that inhabit our world are only a few out of all the inhabitants of all the worlds. Lewis argues that the philosophical utility of modal realism is a good reason for believing that it is true.

How can we talk meaningfully about the past if it does not exist to be talked about? What gives time its direction? Is time travel possible? This defence of presentism - the view that only the present exists - makes an original contribution to a fast growing and exciting debate.

This paper suggests that time travelling scenarios commonly depicted in science fiction introduce problems and dangers for the time traveller. If time travel takes time, then time travellers risk collision with past objects, relocation to distant parts of the universe, and time travel-specific injuries. I propose several models of time travel that avoid the dangers and risks of time travel taking time, and that introduce new questions about the relationship between time travel and spatial location.

And the first step of the Peripatetick argument is that, where Aristotle proveth the integrity and perfection of the World, telling us, that it is not a simple line, nor a bare superficies, but a body adorned with Longitude, Latitude and Profundity; and because there are no more dimensions but these three; the World having them, hath all, and having all, is to be concluded perfect. And again, that by simple length, that magnitude is constituted, which is called a line, (...) to which adding breadth, there is formed a Superficies, and yet further adding the altitude or profundity, there results the Body, and after these three the dimensions there is no passing farther, so that in these three the integrity, and to so speak, totality is terminated, which I might but with justice have required Aristotle to have proved to me by necessary consequences, the rather in regard he was able to do it very plainly and speedily. *Received 9. iii. 54. (shrink)

This is an expansion of the author's 1991 work which investigates the implications of Gödel's writings on Einstein's theory of relativity as they relate to the fundamental questions of the nature of time and the possibilities for time travel.

Might time be multi‐dimensional? In exploring this question, this paper uses a thought‐experiment about dimensionality, H. G. Wells' ‘The Plattner Story’. Plattner has his left and right sides transposed after a trip through a fourth spatial dimension, a change with independent empirical consequences. This example is then generalised to reversals of the directions of time and entropy. Finally, this thought‐experiment is related to relativistic theories of time and the possibility of preserving causality in a temporally multi‐dimensional framework.

This idea of time travel has long given philosophers difficulties. Most recently, in his paper ‘Troubles with Time Travel’ William Grey presents a number of objections to time travel, some well known in the philosophical literature, others quite novel. In particular Grey's ‘no destinations’ and ‘double occupation’ objections I take to be original, while what I will call the ‘times paradox’ and the ‘possibility restriction argument’ are versions of well known objections. I show how each of these can be answered, (...) thereby defending the plausibility of time travel. (shrink)

Identities formed with proper names may be contingent. this claim is made first through an example. the paper then develops a theory of the semantics of concrete things, with contingent identity as a consequence. this general theory lets concrete things be made up canonically from fundamental physical entities. it includes theories of proper names, variables, cross-world identity with respect to a sortal, and modal and dispositional properties. the theory, it is argued, is coherent and superior to its rivals, in that (...) it stems naturally from a systematic picture of the physical world. (shrink)

When Russell was fifteen, he was given a copy of W. K. Clifford’s _The Common Sense of the Exact Sciences_ (1886). Russell later recalled reading it immediately “with passionate interest and with an intoxicating delight in intellectual clarification”. Why then, when Russell wrote _An Essay on the Foundations of Geometry_ (1897), did he choose to defend spaces of homogeneous curvature as a priori? Why was he almost completely silent thereafter on the subject of Clifford, and his writings on geometry and (...) space? We suggest that Russell may have avoided Clifford’s hypothesis that space had heterogeneous curvature because it seemed impossible to reconcile a coherent theory of measurement with a space of variable curvature. Whitehead objected to Einstein’s general theory of relativity on this basis, formulating an alternate theory that preserved the constant curvature of space and, therefore, a familiar sense of measurement. After Einstein’s general theory, Russell chose to distance himself from the position he argued in the _Essay_. (shrink)

This paper sketches a taxonomy of forms of substantivalism and relationism concerning space and time, and of the traditional arguments for these positions. Several natural sorts of relationism are able to account for Newton's bucket experiment. Conversely, appropriately constructed substantivalism can survive Leibniz's critique, a fact which has been obscured by the conflation of two of Leibniz's arguments. The form of relationism appropriate to the Special Theory of Relativity is also able to evade the problems raised by Field. I survey (...) the effect of the General Theory of Relativity and of plenism on these considerations. (shrink)

Talk about time travel is puzzling even if it isn't obviously contradictory. Philosophers however are divided about whether time travel involves empirical paradox or some deeper metaphysical incoherence. It is suggested that time travel requires a Parmenidean four-dimensionalist metaphysical conception of the world in time. The possibility of time travel is addressed (mainly) from within a Parmenidean metaphysical framework, which is accepted by David Lewis in his defence of the coherence of time travel. It is argued that time travel raises (...) formidable difficulties which are not satisfactorily resolved by Lewis's ingenious defence of time travel. Objections to time travel considered include: (1) travel to other times is impossible because there is nowhere (or “nowhen”) to go to; (2) the problem that upon setting out on a journey to the past a time machine will collide with itself; (3) time travel generates a mysterious temporal dualism between experiential time and physical time; (4) travel to the past permits reverse causation, raising the possibility of causal loops and attendant problems arising, for example, from the prospect of empirical contradiction and the possibility of someone being one of their ancestors. (shrink)

This is the first reprint, complete with a new introduction by John Slater.

"Clifford was famous for his public lectures on physics and math and ethics because he explained complex things with easily understood, concrete examples. As you read through his clear, simple explanations of the true bases of number, algebra and geometry you will find yourself getting angry and saying "Why the hell wasn't I taught math this way?" and "Do math ed professors know so little mathematics that they have never heard of Clifford.?" Clifford was destined to be England's Einstein until (...) his untimely death at the age of 34, just 11 days before Einstein's birth. More than 30 years before the Special Theory of Relativity was proposed he had already concluded that the force of gravity was actually due to changes in the curvature of space. He gives explanatory examples in this book that middle school children can understand."--review on Amazon.com viewed July 6, 2020. (shrink)