Taken in its widest sense, the essence of a thing (οὐσία, _essentia_, τὸ τί ἐστι, _quod quid est_, _quidditas_) means _that by which a thing is what it is_: _id quo res est id quod est_: that which gives us the answer to the question, What is this thing? _Quid est haec res?_ τί ἐστι τόδε τι.(84) Now of course any individual thing is what it is just precisely by all the reality that is in it; but we have no direct or intuitive intellectual insight into this reality; we understand it only by degrees; we explore it from various points of view, abstracting and generalizing partial aspects of it as we compare it with other things and seek to classify and define it: _ratio humana essentias rerum quasi venatur_, as the scholastics say: the human mind hunts, as it were, after the essences or natures of things. Understanding the individual datum of sense experience (what Aristotle called τόδε τι, or οὐσία πρώτη, and the scholastics _hoc aliquid_, or _substantia prima_), _e.g._ this individual, Socrates, first under the vaguest concept of being, then gradually under the more and more determinate concepts of substance, corporeal, living, sentient, rational, it finally forms the complex concept of his _species infima_, expressed by his lowest class-name, “man,” and explicitly set forth in the definition of his specific nature as a “rational animal”. Nor does our reason fail to realize that by reaching this concept of the _specific_ essence or nature of the individual, Socrates, it has not yet grasped all the reality whereby the individual is what he is. It has reached what he has in common with all other individuals of his class, what is essential to him _as a man_; it has distinguished this from the unanalysed something which makes him _this particular individual_ of his class, and which makes his specific essence this _individual essence_ (_essentia_ “_atoma_,” or “_individua_”); and it has also distinguished his essence from those accidental and ever varying attributes which are not essential to him as a man, and from those which are not essential to him as Socrates. It is only the unfathomed individual essence, as existing _hic et nunc_, that is _concrete_. All the mind’s generic and specific representations of it—_e.g._ of Socrates as a corporeal substance, a living being, a sentient being, a rational animal—are _abstract_, and all more or less inadequate, none of them exhausting its knowable reality. But it is only in so far as the mind is able to represent concrete individual things by such abstract concepts, that it can attain to _intellectual_ knowledge of their nature or reality. Hence it is that by the term “essence,” simply and _sine addito_, we always mean the essence as grasped by abstract generic or specific concepts (ἔιδος, _species_), and as thus capable of definition (λόγος, _ratio rei_). “The essence,” says St. Thomas, “is that by which the thing is constituted in its proper genus or