"_Penser c'est sentir_," said Condillac. "It is evident," said Bishop Berkeley, "to one who takes a survey of the _objects_ of Human Knowledge that they are either ideas actually imprinted on the senses or else such as are perceived by attending to the passions and operations of the Mind, or lastly ideas formed by help of memory and imagination either combining, dividing, or barely representing those originally perceived in the foresaid ways." J. S. Mill tells us, "The points, lines, circles, and squares which one has in his mind are, I apprehend, simply copies of points, lines, circles, and squares which he has known in his experience," and again, "The character of necessity ascribed to the truths of Mathematics and even, with some reservations to be hereafter made, the peculiar certainty attributed to them is an illusion." "In the case of the definitions of Geometry there exist no real things exactly conformable to the definitions." Again Taine, "_Les images sont les exactes reproductions de la sensation._" Again Diderot, "_Pour imaginer il faut colorer un fond et détacher de ce fait des points en leur supposant une couleur differente de celle du fond. Restituez à ces points la même couleur qu'au fond,--à l'instant ils se confondent avec lui et la figure disparait_," etc. Again, Dr. Ernest Mach, Vienna, remarks, "We are aware of but one species of elements of Consciousness: sensations." "In our perceptions of Space we are dependent on sensations." Dr. Mach repeatedly refers to "space-sensations," and indeed affirms that all sensation is spatial in character.[18:1] According to the view of Knowledge of which we have extracted examples above, the ideas of the mind are originally furnished to it by sensation, from which therefore are derived, not necessarily all our Thoughts, but all the materials of Discourse, all that constitutes the essence of Knowledge. Our purpose at the moment is to show that this view is altogether false, and our counter proposition is, that it is from our Activity that we derive our fundamental conceptions of the external world; that sensations only mark the interruptions in the dynamic Activity in which we as potent beings partake, and that they serve therefore to denote and distinguish our Experience, but do not constitute its essence. We do not propose now to devote any time to the work of showing that sensations from their very nature could never become the instruments of Knowledge. We propose rather to turn to the principal ideas of the external world which are the common equipment of the Mind in order to ascertain whether in point of fact they are derived from Sensation.