Wilhelm Wundt, answered from the texts and cited to the page.
Apperception is the act by which the mind seizes a content and raises it into the clear focus of consciousness — not merely the passive registration that brings something above the threshold of awareness, but the active grasping that places it within the narrower threshold of attention.1 The distinction matters: apprehension is the entrance of content into consciousness, the dim awareness that something is present; apperception is the further act, the attention that takes hold of it and makes it distinct.2
These are not two separate faculties but two regions of one continuous process, the one shading into the other. What makes apperception the central concept of my psychology is its volitional character. Feelings are the invariable forerunners of volition, indicating the direction an act of will shall follow before it has itself become conscious; and precisely these same feelings attend the process of apperception.3
The act of apperception, whether it arises passively under the constraining pressure of an external stimulus or actively through the selective preference of one content over competing others, shows in both cases all the marks of a volitional act.4 Passive apperception corresponds to the impulsive act, in which a single motive carries the process without conflict; active apperception corresponds to the act of choice, in which competing contents press upon attention and the decision is often prefaced by a perceptible strife between motives.5
The older psychology spoke of "involuntary" and "voluntary" attention as though these were different in kind; but in reality volitional processes are present in both — they differ only in grade, as the impulsive act differs from deliberate choice, not as will differs from something other than will.6 From this it follows that apperception is the one original act of will.
It can exist without the external consequences that follow upon other volitional acts, whereas every external act presupposes some prior internal act of apperception as its condition.7 And since apperception accompanies every process of consciousness, always changing yet always present, it forms the lasting substratum of self-consciousness — what we call the ego is neither an idea nor a specific feeling but precisely these elementary volitional processes of apperception, the innermost core of what each of us means when we say "I."8
we understand by apprehension simply the entrance of some content into consciousness an entrance that can be in fact proved, and by apperception the grasping of this by the attention. The apprehended content is that of which we are more or less darkly aware... The apperceived content is that of which we are clearly aware, or, keeping to the figure of speech of a threshold, that which lies above the narrower threshold of attention.An Introduction to Psychology
we understand by apprehension simply the entrance of some content into consciousness an entrance that can be in fact proved, and by apperception the grasping of this by the attention.An Introduction to Psychology
Feelings we found to be invariable forerunners and concomitants of volition. They indicate the direction which an act of will will follow before it has itself become conscious; and when it has attained its full force, they are still present to colour and explicate its effect.Lectures on Human and Animal Psychology
ideas and the feelings that are connected with them serve as motives to the act of apperception, while apperception itself shows all the characteristics of an act of will.Lectures on Human and Animal Psychology
passive apperception corresponds to a simple impulsive act, while the active form corresponds to a complex voluntary act... in the case of active apperception, other psychical contents with their affective elements tend to force themselves upon the attention during the preparatory affective stages, so that the act of apperception when it finally is performed is often recognized as a voluntary process or even as a selective process when the conflict between different contents comes clearly into consciousness.Outlines of Psychology
These have often been called processes of involuntary and voluntary attention. But these expressions are unsuitable, since in reality volitional processes are present in both cases. They are, like impulsive and voluntary acts, merely processes of different grades.An Introduction to Psychology
apperception is the one original act of will. It can exist without the consequences which follow upon other acts of will, whereas these always presuppose as their condition some internal act.Lectures on Human and Animal Psychology
this ego is neither an idea, nor a specific feeling, but it consists of those elementary volitional processes of apperception which accompany the processes of consciousness. They are always changing but they are always present, and in this way form the lasting substratum of our self-consciousness.An Introduction to Psychology