Discussion of the distinction between what is essential and what is merely necessary

Second Principle

The point at issue is that the difference they posit between an accidental property that necessarily accompanies a quiddity and an essential attribute is in fact illusory. Evenness and oddness in relation to an even or odd number stand exactly where rationality and neighing stand with respect to the animal—man in the first case, the horse in the second. Whenever either the subject or the predicate comes to mind, it is impossible to conceive the subject apart from the predicate.

Should someone object that four or three can be envisaged without simultaneously thinking of that number as even or odd, we reply that one can likewise call to mind “man” without at once recalling that he is rational, or even that he is an animal. If it is then said that such a notion falls short of a complete conception of man, we answer that the same holds for one’s conception of four or three.

The same reasoning applies to the accident we call blackness. If blackness is imagined without one’s noticing that it is a colour, or without calling to mind that it is an accident, a quality subsisting in something else, it is still impossible to picture blackness—or whiteness—while supposing that it does not inhere in another. Whenever the idea occurs, one must know that it subsists in something else. Similarly, when a sentient, growing body that moves by volition is brought to mind along with “man,” one must recognize that man is characterised by those traits—indeed, the cognitive tie in the mind is even stronger in the case of colour.

To declare, therefore, that these inseparable attributes are nonetheless not essential, whereas certain others are essential to man, is pure arbitrariness. If any distinction could be upheld, the opposite judgement would be closer to the truth.

For that very reason they contradict themselves in their own examples. They maintain that animality is essential to an animal—whether man, horse, or anything else—so that “animal” is the shared essential, while “rational” is