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?"

In this single expression a whole train of facts are taken into the
account, tho not particularly marked. As a single expression we imply
that _trees increase their stature_. But this we all know could never be
effected without the influence of other causes. The soil where it stands
must contain properties suited to the _growth_ of the tree. A due
portion of moisture and heat are also requisite. These facts all exist,
and are indispensable to make good the expression that the "tree grows."
We might also trace the capabilities of the tree itself, its roots,
bark, veins or pores, fibres or grains, its succulent and absorbent
powers. But, as in the case of the "man that killed the deer," noticed
in a former lecture, the mind here conceives a single idea of a complete
whole, which is signified by the single expression, "trees grow."

Let the following example serve in further illustration of this point.
Take two bricks, the one heated to a high temperature, the other cold.
Put them together, and in a sho