The ontological argument appears in a multiplicity of forms. Over the past ten or twelve years, however, the philosophical community seems to have been concerned principally with those versions of the proof which claim that God is a necessary being. In contemporary literature, Professors Malcolm and Hartshorne have been the chief advocates of this view, both men holding that God must be conceived as a necessary being and that, as a result, his existence is able to be demonstrated a priori. This claim has not gone unchallenged; indeed, numerous writers have argued that neither Malcolm nor Hartshorne has exercised due care in his use of ‘necessary’. That is, critics charge that the arguments of both men have only the appearance of validity, for in their reasonings the defenders of the a priori proof have tacitly assumed that God is a logically necessary being. Whether or not a being can be logically necessary, however, is a quaestio disputata. In fact, until recently the question was not in dispute at all—virtually all ‘competent judges’ agreed that only propositions could be spoken of as logically necessary, and thus that God must be defined as a physically or factually necessary being. But is the statement, ‘a physically necessary being exists’, logically true? Critics of the ontological argument think not; and in support of this view they offer analyses of ‘physical necessity’ which, they feel, not only give meaning to the phrase, but also show that a physically necessary being's existence can be proven only by some kind of a posteriori investigation.