It might at first seem as if perception, introspection, and memory must exhaust all that is meant by immediate, or self-evident, knowledge, and as if what I have here called belief must be uniformly mediate, derivate, or inferred knowledge. The apprehension of something now present to the mind, externally or internally, and the reapprehension through the process of memory of what was once so apprehended, might appear to be the whole of what can by any stretch of language be called direct cognition of things. This at least would seem to follow from the empirical theory of knowledge, which regards perception and memory as the ground or logical source of all other forms of knowledge. And even if we suppose, with some philosophers, that there are certain innate principles of knowledge, it seems now to be generally allowed that these, apart from the particular facts of experience, are merely abstractions; and that they only develop into complete knowledge when they receive some empirical content, which must be supplied either by present perception or by memory. So that in this case, too, all definite concrete knowledge would seem to be either presentative cognition, memory, or, lastly, some mode of inference from these. A little inquiry into the mental operations which I here include under the name belief will show, however, that they are by no means uniformly process of inference. To take the simplest form of such knowledge, anticipation of some personal experience: this may arise quite apart from recollection, as a spontaneous projection of a mental image into the future. A person may feel "intuitively certain" that something is going to happen to him which does not resemble anything in his past experience. Not only so; even when the expectation corresponds to a bit of past experience, this source of the expectation may, under certain circumstances, be altogether lost to view, and the belief assume a secondarily automatic or intuitive character. Thus, a man may have first entertained a belief in the success of some undertaking as the result of a rough process of inference, but afterwards go on trusting when the grounds for his confidence are wholly lost sight of. This much may suffice for the present to show that belief sometimes approximates to immediate, or self-evident, conviction. How far this is the case will come out in the course of our inquiry into its different forms. This being so, it will be needful to include in our present study the errors connected with the process of belief in so far as they simulate the immediate instantaneous form of illusion.