ither of them: a man eats
bread, and it becomes muscle, nerve and bone. In such cases we cannot
trace the qualities of the causal agents in the qualities of the
effects; given such causes, we can prove experimentally, according to
the canons of induction, that they have such effects; but we may not be
able in any new case to calculate what the effects will be.

On the other hand, in Astronomy and Physics, the causes treated of are
mechanical; at least, it is the aim of Physics to attain to a mechanical
conception of phenomena; so that, in every new combination of forces,
the intermixed effect, or resultant, may be calculated beforehand;
provided that the forces concerned admit of being quantitatively
estimated, and that the conditions of their combination are not so
complex as to baffle the powers of mathematicians. In such cases, when
direct observation or experiment is insufficient to resolve an effect
into the laws of its conditions, the general method is to calculate
what may be expected from a combination of its conditions, as either
known or hypothetically assumed, and to compare this anticipation with
the actual phenomenon.

Sec. 3. This is what Mill calls the Direct Deductive Method; or, the
Physical Method, because it is so much relied on in treating of Light,
Heat, Sound, etc.; it is also the method of Astronomy and much used in
Economics: Deduction leads the way, and its results are tested
inductively by experiments or observations. Given any complex mechanical
phenomenon, the inquirer considers--(1) what laws already ascertained
seem likely to apply to it (in default of known laws, hypotheses are
substituted: _cf._ chap. xviii.); he then--(2) computes the effect that
will follow from these laws in circumstances similar to the case before
him; and (3) he verifies his conclusion by comparing it with the actual
phenomenon.

A simple example of this method is the explanation of the rise of water
in the 'common pump.' We know three laws applicable to this case: (a)
that the atmosphere weighs upon the water outside the pump with a
pressure of 15 lb. to the square inch; (b) that a liquid (and therefore
the water) transmits pressure equally in all directions (upwards as well
as downwards and sideways); and (c) that pressure upon a body in any
direction, if not counteracted by an opposite pressure, produces motion.
Hence, when the rise of the piston of the pump removes the pressure upon
the water within the cylind