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A simple remark on the principles of relative action, and we will pass to the consideration of agents and objects, or the more immediate causes and effects of action.

We go forth at the evening hour and look upon the sun sinking beneath the horizon; we mark the varying hues of light as they appear, and change, and fade away.  We see the shades of night approaching, with a gradual pace, till the beautiful landscape on which we had been gazing, the hills and the meadows; the farm house and the cultivated fields, the grove, the orchard, and the garden; the tranquil lake and the babbling brook; the dairy returning home, and the lambkins gambolling beside their dams; all recede from our view, and appear to us no longer.  All this is relative action.  But so far as language and ideas are concerned, it matters not whether the sun actually sinks behind the hills, or the hills interpose between it and us; whether the landscape recedes from our view, or the shades of night intercept so as to obscure our vision.  The habit of thought is the same, and the form of expression must agree with it.  We say the sun rises and sets, in reference to the obvious fact, without stopping to inquire whether it really moves or not.  Nor is such an inquiry at all necessary, as to matter of fact, for all we mean by such expressions, is, that by some process, immaterial to the case in hand, the sun stands in a new relation to the earth, its altitude is elevated or depressed, and hence the action is strictly relative.  For we should remember that rising and setting, up and down, above and below, in reference to the earth, are only relative terms.

We speak and read of the changes of the moon, and we correctly understand each other.  But in truth the moon changes no more at one time than at another.  The action is purely relative.  One day we observe it before the sun, and the next behind it, as we understand these terms.  The precise time of the change, when it will appear to us in a different relation to the sun, is computed by astronomers, and set down in our almanacs; but it changes no more at that time than at any other, for like every thing else, it is always changing.

In a case we mentioned in a former lecture, “John looks like or resembles his brother,” we have an example of relative action.  So in the case of two men travelling the same way, starting together, but advancing at different rates; one, we say, falls behind the other.  In this manner of expression, we follow exactly the principles on which we started, and suit our language to our ideas and habits of thinking.  By the law of optics things are reflected upon the retina of the eye inversely, that is, upside down; but they are always seen in a proper relation to each other, and if there is any thing wrong in the case, it is overcome by early habit; and so our language accords with things as they are manifested to our understandings.