Vagueness comes in various kinds. In the first place, everything that human beings think or say is vague in principle, because of the weakness of human understanding. Our understanding is to the fullness of reality “as the eyes of bats are to the blaze of day,” and this weakness of our understanding is found within everything we think or say. Discussing why there is something rather than nothing, we saw that there must be a being which is necessary in itself. But even after making this argument, this does not become evident to us in itself, and this is because we do not know the nature of being.
Considering whether names said of God belong primarily to him or to us, St. Thomas says,
I answer that, In names predicated of many in an analogical sense, all are predicated because they have reference to some one thing; and this one thing must be placed in the definition of them all. And since that expressed by the name is the definition, as the Philosopher says (Metaph. iv), such a name must be applied primarily to that which is put in the definition of such other things, and secondarily to these others according as they approach more or less to that first. Thus, for instance, “healthy” applied to animals comes into the definition of “healthy” applied to medicine, which is called healthy as being the cause of health in the animal; and also into the definition of “healthy” which is applied to urine, which is called healthy in so far as it is the sign of the animal’s health. Thus all names applied metaphorically to God, are applied to creatures primarily rather than to God, because when said of God they mean only similitudes to such creatures. For as “smiling” applied to a field means only that the field in the beauty of its flowering is like the beauty of the human smile by proportionate likeness, so the name of “lion” applied to God means only that God manifests strength in His works, as a lion in his. Thus it is clear that applied to God the signification of names can be defined only from what is said of creatures. But to other names not applied to God in a metaphorical sense, the same rule would apply if they were spoken of God as the cause only, as some have supposed. For when it is said, “God is good,” it would then only mean “God is the cause of the creature’s goodness”; thus the term good applied to God would included in its meaning the creature’s goodness. Hence “good” would apply primarily to creatures rather than to God. But as was shown above, these names are applied to God not as the cause only, but also essentially. For the words, “God is good,” or “wise,” signify not only that He is the cause of wisdom or goodness, but that these exist in Him in a more excellent way. Hence as regards what the name signifies, these names are applied primarily to God rather than to creatures, because these perfections flow from God to creatures; but as regards the imposition of the names, they are primarily applied by us to creatures which we know first. Hence they have a mode of signification which belongs to creatures, as said above.
St. Thomas does not say it explicitly, but the principle he presents here, “this one thing must be placed in the definition of them all,” implies that according to his argument, God is contained in the definitions of creatures. And in this way the being which is necessary in itself must be in the definition of every being. Consequently our failure to understand the nature of being in itself implies a failure to fully understand the nature of any being.
Second, someone can say something vague for the sake of accuracy, namely because he knows that his knowledge is vague, and he wishes to express his knowledge as it is, rather than claiming to know more than he does. Freeman Dyson and others say that “it is better to be wrong than vague,” but they are, strictly speaking, wrong about this. It is better to be vague and right, rather than clear and distinct, but wrong.
Third, someone can say something vague because he does not primarily care about whether or not it is true, but about something else. In this case it may be left vague because there is no need for the effort that it would take to make it clear, since the person’s purposes can be achieved without that effort, or it may be left vague because those purposes are achieved even better when it remains vague. Thus there is a story about Hegel, which I was unable to track down while writing this post, which says that he explained to someone that the Phenomenology of Spirit had to be extremely difficult to understand, in order to ensure that Hegel would become famous. I do not find the story particularly credible, but I do find the motive credible.
I said “strictly speaking” above in discussing people who say that it is better to be wrong than vague, because in many cases such people are actually opposing the third kind of vagueness, and are simply intending to say that it is better to care about the truth but to make a mistake, than not to care about the truth at all, and therefore not to bother to say something which could turn out to be wrong.