Note: Most of the ideas here are developed further in my 2018 book, Philosophy of Time and Perceptual Experience.
The metaphysical condition on illusion means that, if there is an illusion, then the world is a certain way. There are certain things that are real in the world, certain others that are not. The appearances in cases of illusion then fail to match this way the world is: either it seems as if (a) things are real which are not real or (b) things are unreal which are real . For the sake of this discussion from now, I’ll normally talk only about cases of (a): where it seems as if things are real which are not real. There are one or two exceptions to this, as will turn up as we go along .
So, this seems right:
1. If x is unreal, and it appears as if x is real, i.e., there is a discrepancy between the appearance and reality of x, then there is an illusion of x.
At this stage, we can introduce the broad relevance of the metaphysics of time.
It is this: there are contemporary debates in the metaphysics of time about the reality of things, events and properties in time. Which side you fall on in this debate at least partially determines what you hold to be real (it does not wholly do it, of course; there will be other factors in what you hold to be real, as I’ll come to). Thus, one can say this initially: That there is a discrepancy between x’s reality, and the appearance of x’s reality, may be partially determined by the reality of x in time. And so:
2. That there is a discrepancy between x’s reality, and the appearance of x’s reality, may be determined by which metaphysical position of time is correct.
1. I prefer to speak this way even though it raises the, to some, thorny issue of referring to things that are unreal. I could translate ‘there are things that are unreal’ to some sentence which does not even seem to refer to ‘things’. This is an issue in philosophy but not one I feel I need resolve. Similarly, I talk about things that seem real but are not real. ‘Things’ could refer to objects or they maybe refer to properties (as will be seen, I even use it to cover events). Philosophers tend to draw a line between illusions and hallucinations based on whether or not it is objects or properties that seem to be the case (or real) but are not the case (or real). This distinction is important, later, but I don’t want to make it here, for now.
2. E.g., the Perky effect, a common interpretation of which is that subjects interpreted what they actually see as merely being visually imagined. In my 2011b, I argue that this is an example of illusion in the experience of sensory imagining, and so not perceptual illusion. I think some will think this is not right, because, e.g., the only experience of which there can be illusions are perceptions of properties belonging to external things; I discuss this in the section on Conditions of Illusion II.
There is another reason one might reject this example as evidence of such illusion. From recent conference and personal discussion, I’ve become aware that the set-up and even basic interpretation of the Perky effect is being called into question (I’m thinking here of discussion in two workshops in Glasgow in 2011, one on cross-modal effects (in March 2011) and one on ‘Imagination and Memory’ (in September 2011)). I think this is less important: it may be that there is no empirical evidence of this occurring to us, and so no actual cases of mistaken imaginings. Every time we have an experience that seems to be imagining some x, it is imagining x; there are no erroneous cases. Still, my point here is: if there were such cases, because there is a discrepancy between appearance and reality, they would be illusions. (For other very recent philosophical discussions about the Perky Effect, see Hopkins 2012 and Nanay 2012’s reply to Hopkins (in the same issue)).