ities throughout all phenomena, in the movements of the heavenly
bodies and in the living substance of the cell alike, is the primary aim
of all investigation. We are still far away from this goal, and it is more
than questionable whether we shall ever reach it.

But if the goal should ever be reached, if, in other words, we should ever
be able to say with certainty what must result if occurrences _a_ and _b_
are given, or what _a_ and _b_ must have been when _c_ occurs, would
explanation then have taken the place of description? Or would
understanding have replaced mystery? Obviously not at all. It has indeed
often been supposed that this would be the case. People have imagined they
have understood, when they have seen that "that is always so, and that it
always happens in this particular way." But this is a naive idea. The
region of the described has merely become larger, and the riddle has
become more complex. For now we have before us not only the things
themselves, but the more marvellous laws which "govern" them. But laws are
not forces or impelling causes. They do not cause anything to happen, and
they do not explain anything. And as in the case of things so in that of
laws, we want to know how they are, whence they come, and why they are as
they are and not quite different. The fact that we have described them
simply excites still more strongly the desire to explain them. To explain
is to be able to answer the question "Why?"

Natural science is very well aware of this. It calls its previous
descriptions "merely historical," and it desires to supplement these with
aetiology, causal explanation, a deeper interpretation, that in its turn
will make laws superfluous, because it will penetrate so deeply into the
nature of things that it will see precisely why these, and not other laws
of variation, of development, of becoming, hold sway. This is just the
meaning of the "reductions" of which we have already spoken. For instance,
in regard to crystal formation, "explanation" will have replaced
description only when, instead of demonstrating the forms and laws
according to which a particular crystal always and necessarily arises out
of a particular solution, we are able to show why, from a particular
mixture and because of certain co-operating molecular forces, and of other
more primary, more remote, but also intelligible conditions, these forms
and processes of crystallisation should always and of necessity occur. If
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