In the external world there is no single entity that embraces all entities, nor any existence that encompasses every existent; such comprehensive notions dwell only in the mind.
It is well known—among them and others—that universals exist as universals solely in the intellect, not in concrete reality. They classify these universals into three: natural, rational, and logical.
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The natural (ṭabīʿī) universal is the universal taken absolutely, without the added condition of absoluteness. Thus, when “man” or “animal” is considered simply, with no qualification of existence or non-existence, unity or multiplicity, or any other restriction, one is dealing with the natural universal.
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The logical (manṭiqī) universal is that which becomes characterized by generality and particularity, for the logician studies precisely these two aspects. Whether a notion is general or particular is the very subject of logical inquiry.
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The rational (ʿaqlī) universal, a combination of the foregoing, is exemplified by “man” on the proviso that it is both universal and absolute, or by “the absolute” on the proviso of absoluteness. All agree that this exists only in the mind—apart from those who espouse the Platonic Forms.
As for the natural universal, some claim that it also exists outside the mind, while others contest this. The decisive answer is that it does exist externally, but never as a universal. Nothing in the outside world is universal in itself; what the mind conceives universally exists outside only as a determinate, particular, specified, and delimited instance.
When they say, “This exists outside the mind,” their statement is akin to saying of something conceived within the soul that it now exists outside, or to expressions such as, “I acted on what was in your mind,” or, “You voiced what was in your mind.” A thing first exists in itself; then the intellect conceives it; the tongue articulates what the intellect has conceived; and writing records the tongue’s expression.
Hence it is said that a thing possesses four modes of existence: existence in concrete reality, existence in the intellect, existence in speech, and existence in writing—that is, physical, cognitive, verbal, and written existence.