Ideas of Time
It is well known that Einstein was influenced by Hume and Mach (who was also influenced by Hume). In particular, his treatment of simultaneity was influenced by Hume’s discussion of ideas of time in the Treatise. So it might be useful to revisit that discussion to see how well it holds up under scrutiny. Not surprisingly, Hume applies his general theory of ideas to ideas of time, particularly duration: he maintains that our ideas of time derive from our impressions of time. How we conceive time is fixed by how we perceive time. The concept “copies” the percept. Thus we read: “The idea of time, being derived from the succession of our perceptions of every kind, ideas as well as impressions, and impressions of reflection as well as sensation, will afford us an instance of an abstract idea, which comprehends a still greater variety than that of space, and yet is represented in the fancy by some particular individual idea of a determinate quantity and quality”. He asserts: “Nor is it possible for time alone ever to make its appearance, or to be taken notice of by the mind”. Accordingly, “wherever we have no successive perceptions, we have no notion of time”, so that time can only be “discovered by some perceivablesuccession of changeable objects”. Again: “The idea of duration is always derived from a succession of changeable objects, and can never be conveyed to the mind by anything steadfast and unchangeable”. So the idea of duration is always and necessarily that of a perceivable succession of changes in objects and cannot arise without such perceptions. This, as Hume points out, directly follows from his copy theory of ideas, given that we only have impressions of succession not of pure duration. As he says, we have impressions of time only in the form of relations between events—ordering relations. We perceive time passing byperceiving change occurring. So our concept of time is just the concept of such change. For example, we see physical changes in clocks and thereby infer that time is passing: there is no more to our concept of time than what we observe inside and outside.
It is easy to see how this conception of temporal concepts could lead to radical conclusions about simultaneity. If the concept of simultaneity arises purely from impressions of simultaneity, it will turn out to be relative, since what seems simultaneous to one observer will not seem so to another; this is evident from such simple cases as the time lag between impressions of lightning and impressions of thunder, which varies with distance. The concept of absolute simultaneity thus turns out to be meaningless under Hume’s theory of ideas once we take different observers into account. The same is true of duration, since different perceivers will have different rates of perceived succession. In fact, Hume operates with two principles in the Treatise: the copy principle and the principle that what is not contained in an idea so derived is empty. It follows from both together that time itself must conform to our impressions of it—it has no nature independent of such impressions. We thus reach the conclusion that there cannot beduration without the perception of change—the thing itself not just the idea of it. Starkly stated, there is nothing more to time that how it seems to us in perception, for we have no conception of it independently of our perception-derived ideas. Given this philosophy of time, it is not surprising that Einstein motivates and justifies his conception of time by reflecting on the observation of clocks, since these afford our empirical basis for thinking about time. Anything else would violate Hume’s empiricist strictures. Newton’s absolute conception certainly does, since it corresponds to nothing in our experience—no one has ever had an impression of an absolute infinite time independent of change. As Hume says, summing up his position: “The ideas of space and time are therefore no separate or distinct ideas, but merely those of the manner or order, in which objects exist”. It is thus “impossible to conceive either a vacuum and extension without matter, or a time, when there was no succession or change in any real existence”. It is a short step from this to Einstein’s ideas about the relativity of simultaneity and the corollaries he draws from that.
The trouble is that this is all immensely dubious. I will merely list the many objections that have been and can be made against Hume’s theory of ideas. First, there was the missing shade of blue, a straight counterexample to his copy theory. Second, ideas are singular and abstract whereas impressions always include many qualities and are concrete. Third, animals have impressions but don’t generally have corresponding ideas, so the step to concepts cannot consist in an impression leaving a mark in memory. Fourth, elements of images in the imagination are not functionally identical to concepts. Fifth and connected, concepts join together to form propositional thoughts, but impressions don’t (or copies of them): concepts compose but impressions merely co-exist or succeed. Sixth, even images are not really copiesof impressions but have a nature of their own, differing in many ways from impressions.Seventh, Hume relies heavily on the argument that he can think of no other theory of the origin of ideas, but this argument is weak. What about the rationalist theory of innate ideas? What about the theory that ideas are linked more to language than to perception, corresponding to the word-like elements of the lexicon (plausibly regarded as innate)? What about the possibility that we just don’t know the origin of ideas? Better to accept ignorance than push a manifestly inadequate theory. Eighth, what justifies the inference from ideas to reality? Just because our ideas can only have a certain sort of content, why does it follow that reality must reflect this content and contain no more? Isn’t that idealism? Ninth, as Hume recognizes, the very existence of the standard debates about space and time apparently shows that we do have ideas of absolute space and time; it is just that we are arguing about which theory is true.Don’t I have the concept of other minds even though my impression of other minds is purely of the bodily behavior of others? I know quite well what I am talking about; I just don’t know whether other minds really exist. Tenth, concepts are nothing like memories, which they would be if Hume’s theory were correct (they don’t fade or mutate like memories). Eleventh, Hume often conflates ideas of things with ideas of impressions, but most of our ideas are not of impressions: I have an idea of a square thing, but this is not the idea of an impressionof a square thing. The latter idea seems like a reasonable consequence of having an impression, but why should the former be so derived? That would be moving quite beyond the impression itself. Finally, a concept does not present itself to introspection as impression-like: it is far more elusive, hidden, and obscure. We can’t just introspect and report what a concept is, as Hume seems to assume. The theory of concepts is actually quite undeveloped and highly controversial, with Hume’s empiricist theory just one theory among others; it lacks the self-evidence he cheerfully takes it to have.
So Einstein’s reliance on Hume was reliance on a very frail reed—indeed, I would say, a complete non-starter. If this was his basis for rejecting classical notions of time and promoting his own radically revisionary opinions, then it was totally wrongheaded. Humean concept empiricism is not a sound foundation for constructing a theory of the physical world. Nor do I see how Einstein’s arguments for the relativity of simultaneity can survive without such reliance, as inspection of his writings reveals (the Hume-inspired positivism is quite apparent). One is inclined to conclude that bad philosophy leads to bad physics. Certainly, Einstein’s physical theory derives no support from the underlying philosophy of time. It looks as if a false philosophical theory of ideas has led to a bizarre physics full of paradox and puzzlement. The question is whether it is possible to detach STR from the erroneous philosophy that led to it. I can’t see how, but I am no physicist.
Consider this remarkable passage: “If it be a sufficient proof, that we have the idea of a vacuum, because we dispute and reason concerning it; we must for the same reason have the idea of time without any changeable existence; since there is no subject of dispute more frequent and common. But that we really have no such idea, is certain. For whence should it be derived? Does it arise from an impression of sensation or reflection? Point it out distinctly to us, that we may know its nature and qualities. But if you cannot point out any such impression, you may be certain you are mistaken, when you imagine you have any such idea”. Hume’s hectoring use of italics here does nothing to bolster the entirely question-begging character of his rhetorical questions. On the face of it, the existence of the disputes in question provides a simple proof of the falsity of his theory of ideas: how can we be certainwe lack certain ideas when for all the world we are discoursing about them? A rationalist, say, would be unimpressed with Hume’s confidence. What is certain is that we have the ideaof such ideas!
I do agree that Hume’s theory is apt to make an impression on impressionable young minds (if the puns be excused), and he is no doubt a formidable polemicist; but really his empiricist theory of ideas (concepts) is grossly implausible. I wonder whether the young Einstein knew other philosophers’ work as well, particularly the rationalist tradition. He would not be the first or last young person to fall under Hume’s spell. (By the time of the EnquiryHume himself had seen through his earlier exaggerations and doesn’t revisit the topic of time.)