The same idea in a slightly different form is reflected in Avicenna.

The same idea in a slightly different form is reflected in Avicenna. The duality, separation, and alienness in Plato's philosophy has been largely obviated in the philosophies of Aristotle and Avicenna, in which this matter has been based on the well-known Aristotelian theories of hylomorphism and of generation and corruption.

Although Aristotle's theory is most noteworthy for its advantages over its predecessor, especially for its rejection of the spirit-body duality and its advocacy of a kind of real and substantial unity and interrelation of spirit and body, it nonetheless is not devoid of major ambiguities and difficulties. These difficulties pertain to the question of how the natural relation of matter and form is to be depicted and to the question of generation and corruption.

Further steps in the worlds of science and philosophy were necessary if the curtain were to be lifted from over this mystery or if the topic were even to be addressed in a rational and satisfactory way. The precursors to this intellectual and scientific transformation appeared in Europe and created a revolution in the fullest sense of the word. Revolution threw out the good with the bad. All past foundations and structures were cast down at one stroke.

The revolutionaries designed a new scheme for everything. The famous French philosopher, Descartes, articulated a new scheme of spirit-body dualism that in time became the one scheme to accept, reject, or revise. Descartes admitted three realities: God, the soul, and the body. In conceiving that the soul has thought and intelligence but not dimension and the body has dimension but not thought and intelligence, he came to believe that soul and body are separate things.

The objection raised, first by other Europeans, against Descartes's theory is that he had considered only the aspects of duality, difference, and contrast obtaining between spirit and body, but offered no explanation of how spirit and body, which he says represent extremes of disparity and contrast, came to be conjoined. It is important to consider how they connect and are united, what sort of relation obtains between them.

Descartes's theory is in this respect a kind of regression, a reversion to Plato's theory. We seem to be back to the story of the bird and the nest.