According to Aquinas, what is true is precisely what is. A true statement is one that is consistent with things as they are.
If I say that I am six-foot-four and built like a wrestler, that would not be a true statement, because I am neither of those things. If I say that I’m wearing a T-shirt, that is a true statement, because at present I am indeed wearing a T-shirt.
Telling the truth is closely related. I am telling the truth if what I say is consistent with my understanding of the way things are in reality. My statement might or might not be true in fact–that is, I might be mistaken about the way things are. I am lying if my statement is not consistent with my understanding of the way things are. (At least, I am lying if I intend to deceive. If I say, “What a wonderful movie!” in a sarcastic tone of voice about a movie I hate, my statement is false on the face of it, but it would be hard call my statement a lie.)
Language, however, is tricky: we must define our terms with care. Suppose I were to say, “My desk is made of wood.” This is a true statement, but it’s probably misleading. My desk is, in fact, made of particle board and genuine wood veneer (plus screws, plastic fittings, and so forth). Now, wood veneer is wood, and particle board is mostly wood, so I can claim that my statement is true. However, I suspect that by “wood” most of my readers would expect a desk made out of solid pieces of wood, as cut from a tree, not “boards” made of shredded wood chips and sawdust.
Most words have multiple meanings. It is vitally important to use them precisely, and to understand how your interlocutor is using them; this last is part of listening well. There are two things that you especially need to watch out for: the Word With Shifting Meanings, and the Glorious Symbol.
The first happens when a speaker shifts the meaning of a word as his argument progresses. This is not always intentional. In the Open Source Software community, for example, arguments about “free” software led to the common use of the phrases “free-as-in-speech” vs. “free-as-in-beer” to forestall unintentional shifting of meaning. Used intentionally, it’s a potent technique to make a specious argument seem compelling, a kind of verbal bait-and-switch.
The term “Glorious Symbol” is one that I made up for this post; there might be a better word for it. A Glorious Symbol is a word to which is attached a vast cloud of meaning, mostly unspoken, and never precisely defined. (The Symbol might well have a dictionary definition; but that definition isn’t what most users of the word mean.) The Symbol isn’t so much a concept in-and-of-itself as it is a label to which a collection of meanings are attached. Consider these words: Liberal. Conservative. Progressive. Fundamentalist. Spiritual.
Because the words are not clearly defined, each person brings along their own cloud of meanings. Thus, Glorious Symbols can be used to lead groups of people to think they agree with each other when in fact none of them really know what they are agreeing about. Moreover, they are dangerously susceptible to the shifting of meanings. When a word stands for a loose bundle of meanings, and when no one is quite sure just what the bundle comprises, it’s trivially easy to attach a new meaning and claim it was there all along–provided that it’s not too obviously inconsistent with the existing bundle. Then, other meanings can be dropped, and over time the Glorious Symbol can come to mean something entirely different than it had previously.
More later.