To express a thought we use more than a single word, and the words arranged to express a thought we call a sentence.
But there was a time when, through lack of words, we compressed our thought into a single word. The child says to his father, up, meaning, Take me up into your lap; or, book, meaning, This thing in my hand is a book.
These first words always deal with the things that can be learned by the senses; they express the child's ideas of these things.
We have spoken of thoughts and sentences; let us see now whether we can find out what a thought is, and what a sentence is.
A sentence is a group of words expressing a thought; it is a body of which a thought is the soul. It is something that can be seen or heard, while a thought cannot be. Let us see whether, in studying a sentence, we may not learn what a thought is.
In any such sentence as this, Spiders spin, something is said, or asserted, about something. Here it is said, or asserted, of the animals, spiders, that they spin.
The sentence, then, consists of two parts,—the name of that of which something is said, and that which is said of it.
The first of these parts we call the +Subject+ of the sentence; the second, the +Predicate+.
Now, if the sentence, composed of two parts, expresses the thought, there must be in the thought two parts to be expressed. And there are two: viz., something of which we think, and that which we think of it. In the thought expressed by Spiders spin, the animals, spiders, are the something of which we think, and their spinning is what we think of them. In the sentence expressing this thought, the word spiders names that of which we think, and the word spin tells what we think of spiders.
Not every group of words is necessarily a sentence, because it may not be the expression of a thought. Spiders spinning is not a sentence. There is nothing in this expression to show that we have formed a judgment, i.e., that we have really made up our minds that spiders do spin. The spinning is not asserted of the spiders.