All this work of complicated and refined observation, sometimes lasting months and years, results in a monograph containing minute descriptions of organs, of cells, and of intra-cellular structures, the whole represented and defined in words and pictures.

Now, these descriptions and drawings are the display of the various sensations which the zoologist has experienced in the course of his labours; to those sensations are added the very numerous interpretations derived from the memory, reasoning, and often, also, from the imagination on the part of the scholar, the last a source at once of errors and of discoveries.

But everything properly experimental in the work of the zoologist proceeds from the sensations he has felt or might have felt, and in the particular case treated of, these sensations are almost solely visual.
This observation might be repeated with regard to all objects of the outer world which enter into relation with us.

Whether the knowledge of them be of the common-place or of a scientific order matters little.

Sensation is its limit, and all objects are known to us by the sensations they produce in us, and are known to us solely in this manner.