" I think, therefore I am," if taken as an inference is sound because analytical, only repeating in the conclusion, for the sake of emphasis, something assumed in the premise. If taken as an attestation of fact, as I suppose it was meant, it is honest and richly indicative, all its terms being heavy with empirical connotations. What is " thinking," what is "I," what is " therefore," and what is " existence " ? If there were no existence there would certainly be no persons and no thinking, and it may be doubted (as I have indicated above) that anything exists at all. That any being exists that may be called "I," so that I am not a mere essence, is a thousand times more doubtful, and is often denied by the keenest wits. The persuasion that in saying "I am " I have reached an indubitable fact, can only excite a smile in the genuine sceptic. No fact is self-evident ; and what sort of fact is this " I," and in what sense do I " exist" ?
and this inner life of the body, I suspect, was the rock of vulgar belief which Descartes found at hand, easy to mount on, after his not very serious shipwreck. And the rock was well chosen ; not because the existence of my inner man is a simpler or a surer fact than any other ; to a true sceptic this alleged being so busily thinking and willing and fuming within my body is but a strange feature in the fantastic world that appears for the moment. Yet the choice of the inner man as the one certain existence was a happy one, because this sense of life within me is more constant than other perceptions, and not wholly to be shaken off except in profound contemplation or in some strange forms or madness.
The last chapter of Scepticism and Animal Faith is a close analysis of the idealism of Descartes and Hume. It also indicates why Santayana was attracted to Indian philosophy. The method of questioning called Atma Vichara or the probing into the roots of the self by the inquiry who am I was presented by the modern sage Ramana Maharshi (d.1950).Who Am I
Now there is a double bind in this process in that there can be no answer to the question. However that self definition is the context of what Freud took to be the purpose of life namely the creation of a strong central ego. Maya, Sigmund, Maya.
The first extract above is about the process of questioning, the second indicates the goal or at least its partial attainment. The sense of life within me as bare awareness is channelled through a chakra. We in a sense allow ourselves because we are incarnated to fall through that level of awareness. This is generally the Ajna or Hridaya chakra. As Santayana says beyond that is profound formless contemplation.