The truth is the idea of Space is essentially negative. It represents absence of physical obstruction of every kind. No doubt, we may describe it positively as a possibility of free movement, and such a description is at once true and important. Yet even _it_ involves a negative. The term "free" is in reality, though not in form, a negative term and means "unconstrained." And the reason why such a term is necessarily negative is to be found in the fact that a state of dynamic constraint is the essential condition under which we enter upon our organic existence. Freedom is a negation of the Actual. Absolute freedom is a condition only theoretically possible, and is essentially the negation of the state of restraint in which our life is maintained. But the definition last quoted is nevertheless valuable because it clearly shows what really is the origin of the idea of Space. It proves that the idea of Space is a representation of one condition of our Activity. It is because the primary work of Thought is to represent the forms of our dynamic Activity that we find the idea of Space so necessary and fundamental. But it will perhaps be argued that our ordinary sensations carry with them a spatial meaning and implication, and that indirectly, therefore, our sensations _do_ supply us with the idea of Space. It will readily be agreed that if this is so of any sensations it is pre-eminently true of the sensations of vision and touch. Indeed, it will perhaps not be disputed that the ordinary vident man derives from the sensations of vision his most common spatial conceptions. We propose, therefore, to inquire very briefly how the character of spatial extension becomes associated with the data of Vision. The objects of Vision appear to be displayed before us in immense multitude, each distinct from its adjacent neighbour, yet all inter-related as parts of one single whole--the presentation thus constituting what is called Extensity.