One philosophical theory about perception claims that perceiving is inferring the external world from the sensory signals. The argumentation goes as follows. Consider the retina: there is a spatially inhomogeneous set of photoreceptors; the image projected onto the retina is inverted, but you don’t see the world upside down; there are blood vessels that you normally don’t see; there is a blind spot where the optic nerve starts that you normally don’t notice; color properties of photoreceptors and their spatial sampling is inhomogeneous and yet color doesn’t change when you move the eyes. Perceptually, the visual field seems homogeneous and independent of the position of the eyes, apart from a change of perspective. So certainly what you perceive is not the raw sensations coming from your photoreceptors. These raw sensations are indirectly produced by things in the world, which have some constancy (compared to eye movements, for example). The visual signals in your retina are not constant, but somehow your perception is constant. Therefore, so the argument goes, your mind must be reconstructing the external world from the sensory signals, and what you perceive is this reconstruction.
Secondly, visual signals are ambiguous. A classical example is the Necker cube: a wire frame cube drawn in isometric perspective on a piece of paper, which can be perceived in two different ways. More generally, the three-dimensional world is projected on your retina as a two-dimensional image, and yet we see in three dimensions: the full 3D shape of objects must then be inferred. Another example is that in the dark, visual signals are noisy and yet you can see the world, although less clearly, and you don’t see noise.
I would then like to consider the following question: why, when I am looking at an apple, do I not see the back of the apple?
The answer is so obvious that the question sounds silly. Obviously, there is no light going through the object to our eyes, so how come could we see anything behind it? Well precisely, the inference view claims that we perceive things that are not present in the sensory signals but inferred from them. In the case of the Necker cube, there is nothing in the image itself that informs us of the true three-dimensional shape of the cube; there are just two consistent possibilities. But in the same way, when we see an apple, there are a number of plausible possibilities about how the back of the apple should be, and yet we only see the front of the apple. Certainly we see an apple, and we can guess how the back of the apple looks like, but we do not perceive it. A counter-argument would be that inference about the world is partial: of course we cannot infer what is visually occluded by an object. But this is circularly reasoning: perception is the result of inference, but we only infer what can be perceived.
One line of criticism of criticism of the objectivist/inferential view starts from Kant’s remark that anything we can ever experience comes from our senses, and therefore one cannot experience the objective world as such, even through inference, since we have never had access to the things to be inferred. This leads to James Gibson’s ecological theory of perception, who considered that the (phenomenal) world is directly perceived as the invariant structure in the sensory signals (the laws that the signals follow, potentially including self-generated movements). This view is appealing in many respects because it solves the problem raised by Kant (who concluded that there must be an innate notion of space). But it does not account for the examples that motivate the inferential view, such as the Necker cube (or in fact the perception of drawings in general). A related view, O’Regan’s sensorimotor theory of perception, also considers that objects of perception must be defined in terms of relationships between signals (including motor signals) but does not reject the possibility of inference. Simply, what is to be inferred is not an external description of the world but the effect of actions of sensory signals.
So some of the problems of the objectivist inferential view can be solved by redefining what is to be inferred. However, it still remains that in an inferential process, the result of inference is in a sense always greater than its premises: there is more than is directly implied by the current sensory signals. For example, if I infer that there is an apple, I can have some expectations about how the apple should look like if I turn it, and I may be wrong. But this part where I may be wrong, the predictions that I haven’t checked, I actually don’t see it – I can imagine it, perhaps.
Therefore, perception cannot be the result of inference. I suggest that perception involves two processes: 1) an inferential process, which consists in making a hypothesis about sensory signals and their relationship with action; 2) a testing process, in which the hypothesis is tested against sensory signals, possibly involving an action (e.g. an eye movement). These two processes can be seen as coupled, since new sensory signals are produced by the second process. I suggest that it is the second process (which is conditioned by the first one) that gives rise to conscious perception. In other words, to perceive is to check a hypothesis about the senses (possibly involving action). According to this proposition, subliminal perception is possible. That is, a hypothesis may be formed with insufficient time to test it. In this case, the stimulus is not perceived. But it may still influence the way subsequent stimuli are perceived, by influencing future hypotheses or tests.
Update. In The world as an outside memory, Kevin O'Regan expressed a similar view: "It is the act of looking that makes things visible".