Because the context of the essay's titile quote is a course on metaphysics that includes an investigation of various theories of time but not of truth, one is quickly drawn to the assumption that the essay's title is inviting a discussion of Presentism about time. And I will focus this essay on just such a discussion. However, since the title assertion speaks about our evidence and the truth of statements more than it speaks about time, I will also include a few words about the Verification theory of Truth.
Discussions in the philosophy of time are dominated by the two ways of thinking about time identified by J.M.E. McTaggart in his 1908 article "The Unreality of Time"(1). In that landmark article, he identified what he called the "A Series" and the "B Series" basis of the sequencing of events within time.
McTaggart's "A-series" sequences events by making use of our notions of "past, present, and future" (and related English phraseology) resulting in a temporal sequence of events ordered from the far past, through the present, into the far future. His "B-series" sequences events by making use of our notions of "earlier and later" (and related English phraseology) resulting in a temporal sequence of events ordered from earlier moments to later moments.
McTaggart does not suggest that these two different time-series represent different sequences of events within time. Rather they represent two different ways in which the single sequence of events within time can be seen to gain their sequence. It was McTaggart's argument that the A series was a necessary (and in this sense a more fundamental) component of any full theory of time. He reasoned that the B series alone cannot constitute a proper time series because the A series is essential to time.
So called "A-Theorists" maintain that temporal discourse requires the use of tenses - hence they adopt the metaphysical philosophy of time that takes as fundamental McTaggart's A-Series conception of time. Some of these "A Theorists" believe, in addition, that time is fundamentally unlike the dimensions of space. They believe that there are crucial ontological differences between the dimension of time and the dimensions of space. And some of these A Theorists also endorse a view known as "Presentism".
Presentism is that metaphysical (ontological) thesis that maintains that only present things exist. Future and past things are unreal - or at least not as real as present things. Hence an advocate of Presentism would argue (with the essay title) that statements about past and future things can only be true or false in virtue of how things are in the present.
More precisely, Presentism is the view that, necessarily, it is always true that only present objects exist(2). (Note that what is meant here by 'present' is temporally present, and not spatially present.) The ontology of Presentism means that a list of all the things that exist would contain only things that presently exist, and would not contain any thing that does not presently exist. All objects not on that list are unreal, according to Presentism. But Presentists also disagree among themselves about the exact nature of the metaphysical distinction between present things and non-present things. Many Presentists do allow for various degrees of unreality. Sherlock Holmes and Hogwarts are granted a different degree of unreality than are, say, Shakespeare and New York's World Trade Center.
One of the consequences of Presentism is that singular propositions (ie. statements) about non-existent things also do not exist. All singular propositions about things which do not presently exist but did exist in the past (or will exist in the future) do not themselves exist and cannot therefore be true or false. To say that "X is F" is to incur an existential commitment to the existence of "X". If "X" does not exist, then it is improper to state that "X is F". Hence the proposition that "X is F" is invalid, and does not itself exist. (Which is not to suggest that one cannot assert that "X is F". Just that by so asserting one does not make any sense. Unless reinterpreted as suggested by various Presentists, the assertion has no meaning.) Hence, the essay's title assertion that statements about the past can be true or false only in virtue of whatever presently exists - the evidence.
There are two main arguments for Presentism. The first is that the only sensory access we have is to the present. Any information we have about the past or the future is based on information we have in the present.(Our memories of the past are present memories of past events.) If one is going to choose what is real and what is not, it seems to be common sense to identify "The Real" with things that we have access to, relegating "The Unreal" to things that we do not have access to.
The intuitive appeal of Presentism comes from our "common sense" feeling that past things, while they once did exist, no longer do. And that future events, while they might eventually occur, have not yet occurred. Common sense intuition supposedly tells us that the past is fixed and the future not. Presentism provides a natural explanation for this sense that the future is open because it does not yet exist, that time passes in a succession of "Now's, and that past events have slipped irretrievably beyond our access. It is for this reason that Presentism maintains that there is a fundamental metaphysical and ontological difference between time and space.
The other argument, somewhat based on the first, is ontological simplicity. Most philosophers believe that, other things being equal, simpler theories are to be preferred. Ontological simplicity, or parsimony, values the number of kinds of entities accepted by the theory. The default reading of "The Principle of Parsimony" (otherwise known as "Occam's Razor") is as a principle of qualitative parsimony rather than quantitative parsimony. And since it seems to dramatically simplify the kinds of things which we must admit into our ontology if we restrict that list to things in the present, Presentists argue that Presentism is therefore to be preferred.
Balancing these positive appeals of Presentism are the challenges that have been mounted against the position. Despite the claim by many Presentists that theirs is the common sense view, it is pretty clear that they must deal with some major problems. One problem has to do with what appears to be perfectly meaningful talk about non-present objects, such as Shakespeare and the Christmas party we are attending next weekend. If there really are no non-present objects persons, or events, then it is hard to see to what we are referring when we use expressions such as 'Shakespeare' and 'the upcoming Christmas party'. But according to Presentism, there are never any meaningful propositions about non-present objects, and no person ever believes any such proposition. This is surely a strange consequence. As a variation on this problem, consider what happened to Pompeia, wife of Julius Caesar, who was for a time unaware of Caesar's death. Without any discernable change in Pompeia, upon Caesar's death suddenly all of Pompeia's thoughts and beliefs about her husband were rendered meaningless. And upon her husband's death, Pompeia could no longer stand in the relation of being a wife of a man that no longer existed.
Of course, Presentists expend an inordinate amount of prose addressing these problems, and offering solutions of various sorts. Mark Hinchliff(3), for example, distinguishes between "Serious Presentism" and "Unrestricted Presentism". Serious Presentism he claims is the conjunction of Presentism with the claim that an object can have properties, and stand in relations, only when it exists, while Unrestricted Presentism is the conjunction of Presentism with the claim that an object can have properties, and stand in relations, even at times when it does not exist. Haecceities offer a similar solution. According to Markosian(4), the haecceity comes into being when the object comes into being, but to accommodate the presentist, continues to exist after the object ceases to exist. Roderick Chisholm(5), as another example, offers a paraphrasing approach that translates a statement apparently about the properties of non-existent entities into statements about properties of the referent of a descriptor that happens to be empty.
In addition to these problems arising from apparently coherent discussion of non-existent entities, there are also difficulties that arise from the realm of Physics. Einstein's theory of relativity appears to demonstrate that the concept of a universal "Now!" is illusory. Simultaneity is variable depending on the relative velocities of observers. On this basis there is no universally agreed upon "Present" upon which to found Presentism.
Finally, Presentism must address all of the challenges that B-Theorists raise against taking McTaggart's A-Theory as the best basis for understanding temporal sequencing. To all of these various challenges Presentists do offer what they maintain are adequate responses. Yet it remains the intuition of non-presentists that these "solutions" that are being offered are more complex (less parsimonious) than is warranted by the intuition that drives presentism. Whether presentism or non-presentism is the more parsimonious theory is thus up for considerable debate.
The alternative to Presentism is, of course, "Non-Presentism". Non-presentism is an umbrella term that covers several different theories that agree only in denying that the only things that exist are those that are present. One popular version of non-presentism is "Eternalism", which maintains that things (concrete objects, events, persons) from both the past and the future exist just as much as present things. According to Eternalism, non-present objects like Shakespeare and future Christmas parties exist right "now" (in an atemporal understanding of the word), even though they are not currently present. We may not have access to them at the moment, and they may not share the space-time vicinity that we find ourselves in right now, but they are nevertheless on the list of all existing things. According to the Eternalist, temporal location does not matter when it comes to ontology.
But the essay's title statement can also be made by an Eternalist. What an Eternalist would mean by the essay's title statement is that the truth or falseness of statements about the past is determined by present evidence. And this is a statement about the Verificationist theory of truth rather than a statement about the Presentist theory of time. I will address this issue in the next section.
The verificationist theory of truth is an epistemological (or anti-realist) theory of truth that maintains that the truth or falseness of statements or propositions is determined by the evidence we have available to us. Or, in some versions, might have in principle at the limit of investigation. The distinctive claim of verificationism is that the result of such verifications is, by definition, truth. That is, truth is reducible to this process of verification.
Quite independent of the presentism debate described above, the essay's title argument can be seen as a verificationist assertion that all statements about the past are true or false in virtue of the evidence we have of how things were in the past. In other words it can be seen as defining the truth or falseness of statements about the past in terms of the evidence.
There are, of course, many different theories of truth that are offered in the literature as alternatives to verificationism. But since verificationism is an epistemological theory of truth, its most natural alternative is a metaphysical (or realist) theory of truth. The most popular (and the most widely accepted) version is the Correspondence Theory of truth. Although there are a number of such theories, and a number of variations of the Correspondence theory, they all are distinct from verificationism by maintaining that the truth or falseness of statements is evidence transcendent. Statements about the past can be true or false independently of whatever knowledge we may currently have as to how things were in the past.
The essay's title assertion is true only if one adopts a Presentist theory of time (only things in the present exist and can act as truth determinants of statements about the past), and a Verificationist theory of truth (the truth or falseness of statements is determined by the evidence we have - or might have). If either of these theories is rejected, as it would be by an non-presentist about time or a Realist about truth, then the essay's title assertion would have to be judged false.
Since I judge that the driving intuition of presentism is not sufficiently persuasive to overcome all the difficulties with which Presentism is afflicted, and since I judge that our concept of truth is a metaphysical one and not an epistemological one, I reject both presentism and verificationism.I maintain, therefore, that the essay's title assertion is quite false.
(1) McTaggart, J.M.E., The Nature of Existence(2 volumes); Cambridge University Press, Cambridge, England. 1927. ISBN 0-521-35769-1.
(2) Markosian, Ned, "Time", The Stanford Encyclopedia of Philosophy(Winter 2010 Edition), Edward N. Zalta (ed.), URL=<http://plato.stanford.edu/archives/win2010/entries/time/>.
(3) Hinchliff, Mark, "A Defense of Presentism in a Relativistic Setting" in Philosophy of Science, Vol. 67, Supplement. Proceedings of the 1998 Biennial Meetingsof the Philosophy of Science Association. Part II: Symposia Papers (Sep., 2000), pp. S575-S586
"The Puzzle of Change", in Tomberlin, James (ed.), Philosophical Perspectives, 10, Metaphysics (Cambridge, MA: Blackwell Publishers, 1996), pp. 119-136.
(4) Markosian Ned; "A Defense of Presentism" in Oxford Studies in Metaphysics, Volume 1, Dean W.Zimmerman, Dean (ed.), Oxford: Oxford University Press, 2004, pp. 47-82.]
(5) Chisholm, Roderick M., The First Person, University of Minnesota Press, Minneapolis, Minnesota.1981.
On Metaphysics, University of Minnesota Press, Minneapolis, Minnesota. 1989.
"Referring to Things That No Longer Exist" in Philosophical PerspectivesVol 4 (1990), pp. 546-556.
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