In practical talk, a man’s common sense means his good judgment, his freedom from excentricity, his gumption, to use the vernacular word.  In philosophy it means something entirely different, it means his use of certain intellectual forms or categories of thought.  Were we lobsters, or bees, it might be that our organization would have led to our using quite different modes from these of apprehending our experiences.  It might be too (we cannot dogmatically deny this) that such categories, unimaginable by us to-day, would have proved on the whole as serviceable for handling our experiences mentally as those which we actually use.

If this sounds paradoxical to anyone, let him think of analytical geometry.  The identical figures which Euclid defined by intrinsic relations were defined by Descartes by the relations of their points to adventitious co-ordinates, the result being an absolutely different and vastly more potent way of handling curves.  All our conceptions are what the Germans call denkmittel, means by which we handle facts by thinking them.  Experience merely as such doesn’t come ticketed and labeled, we have first to discover what it is.  Kant speaks of it as being in its first intention a gewuehl der erscheinungen, a rhapsodie der wahrnehmungen, a mere motley which we have to unify by our wits.  What we usually do is first to frame some system of concepts mentally classified, serialized, or connected in some intellectual way, and then to use this as a tally by which we ‘keep tab’ on the impressions that present themselves.  When each is referred to some possible place in the conceptual system, it is thereby ‘understood.’  This notion of parallel ‘manifolds’ with their elements standing reciprocally in ‘one-to-one relations,’ is proving so convenient nowadays in mathematics and logic as to supersede more and more the older classificatory conceptions.  There are many conceptual systems of this sort; and the sense manifold is also such a system.  Find a one-to-one relation for your sense-impressions anywhere among the concepts, and in so far forth you rationalize the impressions.  But obviously you can rationalize them by using various conceptual systems.

The old common-sense way of rationalizing them is by a set of concepts of which the most important are these: 

Thing;

The same or different;

Kinds;

Minds;

Bodies;

One Time;

One Space;

Subjects and attributes;

Causal influences;

The fancied;

The real.