We must next review the evidence in its favour, and the objections urged against it.

It is desirable, however, to ask what kind of proof should be expected on such a question.
In the Speculative or Theoretical sciences, we prove a doctrine by referring it to some other doctrine or doctrines, until we come at last to some assumption that must be rested in as ultimate or final.
We can prove the propositions of Euclid, the law of gravitation, the law of atomic proportions, the law of association; we cannot prove our present sensations, nor can we demonstrate that what has been, will be.

The ultimate data must be accepted as self-evident; they have no higher authority than that mankind generally are disposed to accept them.
In the practical Sciences, the question is not as to a principle of the order of nature, but as to an _end_ of human action.

There may be _derived_ Ends, which are susceptible of demonstrative proof; but there must also be _ultimate_ Ends, for which no proof can be offered; they must be received as self-evident, and their sole authority is the person receiving them.

In most of the practical sciences, the ends are derived; the end of Medicine is Health, which is an end subsidiary to the final end of human happiness.