Most people would agree that there are many kinds of necessity – moral, physical, logical and metaphysical being examples. An action would be seen as morally necessary if one’s ethical system required that it be done. An event would be physically necessary if it were determined by the initial conditions and the laws of nature. A conclusion is logically necessary if it is validly entailed by the premises one accepts. Something is metaphysically necessary if it is required by the very nature of being.

As Aristotle points out, terms may be predicated in three ways: univocally (with the identical meaning in each instance), equivocally (with unrelated meanings in the relevant instances) and analogically (with meanings that are partly the same and partly different in the several cases). Clearly, “necessary” does not have the identical meaning in moral and physical contexts; nonetheless, its meanings are related. So, it is analogically predicated.

Medieval logicians distinguished different modes of analogical predication, two being the Analogy of Attribution and the Analogy of Proportionality. The paradigm case of the Analogy of Attribution is the use of “healthy” in the contexts of food, people and urine. Food is not “healthy” in the the same sense that a person is healthy. Rather, healthy food causes, or at least contributes to, personal health. Similarly, healthy urine is not in good health, but a sign of good health. In this type of analogy the base meaning of the term is the same, but the relation of the predicate to that meaning differs. It can be a property, a cause of that property, or a sign of that property. This is not what we see in the analogous predication of “necessity.”

An Analogy of Proportionality occurs when each instance of a term bears the same relation to a differing base. For example, different instances of “good” are linked by an Analogy of Proportionality. The properties that make a road “good” (ample width, smoothness, durability, etc.) are not the properties that make a moral act “good” (justice, proportionality, etc.). Still, each kind of good is “proportional” to what it is predicated of. The properties that make something “good” are those that make it suitable to its purpose.

I suggest that the various meanings of “necessity” are linked by an Analogy of Proportionality. In each instance, something is “necessary” if it is logically entailed by some set of premises, which I’ll call its “basis.” In the first paragraph I noted the basis of several kinds of necessity.

Some consequences of this analysis are:
1. Modal logic need not be a separate branch of logic. Rather modality reflects ordinary deduction restricted by the relevant basis.
2. There is no need for the subjective construct of “possible worlds” to clarify the meaning of modality. Possible Worlds is subjective because no possible world can be observed and experienced other than the one actual world. Rather, possible worlds are imagined worlds that may or may not be logically consistent with respect to the relevant basis. Godel's work suggests that we cannot know which worlds are consistent, and so truly possible, and which are not.
3. The modal signs "□" (necessarily) and "◇" (possibly) can easily obscure equivocations because their meanings are not fixed, but context sensitive, varying in proportion to the relevant basis.