(Or: How the Modern Age turned the tables on Antiquity)
The Modern Age was inaugurated ontologically in the 17th century with the positing of the subject of consciousness, most consummately in the thinking of Descartes. Pronouncing consciousness as the subject already turns the subject (_hypokeimenon_) of ancient Greek thinking on its head, for it was the under-lying _hypo-keimenon_ (from _hypo_ 'under' and _keimai_ 'to lie') that was addressed by the _logos_ of both everyday and philosophical thought.
The guiding question of Greek philosophy, and especially Socrates and Plato, was _ti estin...;_, "What is...?" addressed to the under-lying sub-ject to investigate what it is, i.e. its whatness, quidditas or essence. The question of whatness remains the lead question throughout Antiquity and even into the Christian Middle Ages with their theology investigating what God is.
With Descartes' "cogito ergo sum", "I cogitate/think/feel/sense/perceive, therefore I am", the subject of consciousness is posited. It is not a syllogism with a conclusion, as the word "therefore" misleadingly suggests. There are not even two premises which could be closed together 'syl-logistically' in a con-clusion. Rather, an encapsulated consciousness is posited as the new essence of the subject. This subjective consciousness is confronted with an external world consisting of objects (Gegen-stände) standing over against it.
Antiquity's question, What is the subject (_hypokeimenon_)?, posed in the third person singular is transformed into a positing assertion of certitude, "I am", in the first person, of the Modern Age. The subject of consciousness is posited hermeneuticaly as the fundamentum inconcussum, i.e. the unshakable foundation of encapsulated consciousness, from whose secure base the external world is to be interrogated. This interrogation takes place primarily through scientific theoretical models constructed by consciousness, preferably mathematically, into which experimental data from the external world of objects are fed for testing the theory in question. So long as the theory holds up to experimental verification, this is said to be objective truth.
Who reflects today upon this momentous shift from the third person singular to the first person singular? Who has even noticed it? Why is this first-person subject of consciousness still regarded as some kind of what, especially for scientific purposes, thus perverting first person back into third person? Why is this perversion not clearly seen for what it is? Why has there not been a concurrent shift from the ancient question regarding the whatness of the third-person _hypokeimenon_ to the question regarding the whoness of the first-person subject of consciousness? Why is the very word, 'whoness', or its Latin equivalent, 'quissity', still a strange neologism today? Whereas 'whatness' or 'essence' are accepted as more or less 'natural'?
This state of affairs is not only an oversight due to the growing poverty of thinking in the present age, but also a question of power, of effective power and social power -- a question that is systematically suppressed by institutions of learning.
Further reading: Social Ontology Chaps. 3, 10 and 11.