Our whole knowledge of action depends on an observance of things in a state of motion, or change, or exerting a tendency to change, or to counteract an opposing substance.

This will be admitted so far as material things are concerned.  The same principle holds good in reference to every thing of which we form ideas, or concerning which we use language.  In our definition of nouns we spoke of immaterial and imaginary things to which we gave names and which we consider as agencies capable of exerting an influence in the production of effects, or in resisting actions.  It is therefore unimportant whether the action be real or imaginary.  It is still inseparably connected with the thing that acts; and we employ it thus in the construction of language to express our thoughts.  Thus, lions roar; birds sing; minds reflect; fairies dance; knowledge increases; fancies err; imagination wanders.

This fact should be borne in mind in all our attempts to understand or explain language.  The mind should remain fixed to the acting substance, to observe its changes and relations at different periods, and in different circumstances.  There is no other process by which any knowledge can be gained of actions.  The mind contemplates the acting thing in a condition of change and determines the precise action by the altered condition of the thing, and thus learns to judge of actions by their effects.  The only method by which we can know whether a vegetable grows or not is by comparing its form to-day with what it was some days ago.  We can not decide on the improvement of our children only by observing the same rule.

“By their fruits ye shall know them,” will apply in physics as well as in morals; for we judge of causes only by their effects.  First principles can never be known.  We observe things as they are, and remember how they have been; and from hence deduce our conclusions in reference to the cause of things we do not fully understand, or those consequences which will follow a condition of things as now existing.  It is the business of philosophy to mark these effects, and trace them back to the causes which produced them, by observing all the intermediate changes, forms, attitudes, and conditions, in which such things have, at different times, been placed.

We say, “trees grow.”  But suppose no change had ever been observed in trees, that they had always been as they now are; in stature as lofty, in foliage as green and beautiful, in location unaltered.  Who would then say, “trees grow?”