What the relative values of these different kinds of immediate knowledge are is a point which will have to be touched on at the end of our study.

Here it must suffice to warn the reader against the supposition that this value is assumed to be identical.
It might seem at a first glance to follow from this four-fold scheme of immediate or quasi-immediate knowledge that there are four varieties of illusion.

And this is true in the sense that these four heads cover all the main varieties of illusion.

If there are only four varieties of knowledge which can lay any claim to be considered immediate, it must be that every illusion will simulate the form of one of these varieties, and so be referable to the corresponding division.
But though there are conceivably these four species of illusion, it does not follow that there are any actual instances of each class forthcoming.

This we cannot determine till we have investigated the nature and origin of illusory error.