Charles S. Peirce I begin with the soul of man. For we first learn that brutes have souls from the facts of the human soul. What brutes and other men do & suffer would be quite unintelligible to us, if we had not a standard within ourselves with which to measure others.
Third Lecture
[How can Thought Think of Itself?]
MS 192 (Robin 395): Writings 3, 10-11
Summer-Fall 1872
At the first dawn of cognition we began to compare and consider the objects about us. Our thought first assigned to things their right places and reduced the wild chaos of sensuous impressions to a luminous order. But after thought had classified everything a residuum was left over, which had no place in the classification. This was thought itself. What is this which is left over? After thought has considered everything, it is obliged next to think of itself. Here it is at once means and end. The question is, what is thought,—and the question can only be answered by means of thought.
This is a noticeable circumstance. How can thought think of itself, it is asked; that would be an insoluble contradiction. It is as though a tone should be heard of itself, or a beam of light be seen by itself. But this objection reminds one of the efforts of the man who tried to look at his own eye. After great difficulty he got so far as to see the end of his nose, forgetting that it would be much simpler to hold up a looking-glass to his face. Common sense, which usually hits the nail on the head, has long ago held that looking-glass up to thought. If I wish to represent to myself what thought is, (says common sense) I have only to act as though my thought were an external object which I can consider as I should consider something not a part of myself. Thought thus objectively considered common sense terms the soul . So if we are to investigate in a scientific manner the nature of thought, we //need/can// do nothing else than consider the soul as if it were an object of experience.
Everyone grants that thought is a sort of experience; otherwise, we could not know that we think. Everyone further sees that we have in thought a very varied experience, for it changes both with the object thought of and with mental development which we have attained. Thus, we bring together all the experiences which thought has in itself & subject them to the consideration of our thoughts. There are also other experiences, not properly thoughts, such as sensations and feelings which we term phenomena of the soul, because we recognize them as immediate products of an activity within us, which according to our observation cannot be separated from the activity of thought.