I spoke here about various kinds of necessity, but did not explain the nature of necessity in general. And in the recent post on Hume’s idea of causality, it was not necessary to explain the nature of necessity, because the actual idea of causality does not include necessity. Thus for example a ball can break a window even if it would have been possible for someone to catch the ball, but the person did not do so.
Sometimes it is asked whether necessity implies possibility: if it is necessary that Tuesday follow Monday, it is possible for Tuesday to follow Monday? I am inclined (and I think most are inclined) to say yes, on the grounds that to say that something is not possible is normally understood to imply that the thing is impossible; thus if it is not possible for Tuesday to follow Monday, it is impossible. But this is largely a verbal question: regardless of how we answer this, the real point is that the necessary is the same kind of thing as the possible, except that possibilities are many while the necessary is one. And likewise, a count of zero for the same things implies impossibility. Thus there is something that we are counting: if we find none of them, we speak of an impossibility. If we find only one, we speak of one necessity. And if we find many, we speak of many possibilities.
What are we counting here? Let’s take an example. Horses can be white, or red, or brown, among other possibilities. So there are many possible colors for a horse. And on the other hand snow is always white (or so let us pretend.) So there is only one possible color for snow, and so snow is “necessarily” white. Meanwhile, air is always colorless (or so let us pretend.) So it is impossible for air to have a color. Based on this example, we propose that what we are counting is the number of forms that are suitable for a given matter. Someone might object that if we analyze the word “suitable” here it might involve some sort of circularity. This may well be the case; this is a common occurrence, as with desire and the good, and with virtue and happiness. Nonetheless, I think we will find it worthwhile to work with this definition, just as in those earlier cases.