However beautiful the conception, it must be admitted that in its _a
priori_ aspect, it was not in accordance with human experience and
analogy to anticipate a successful issue. In nature law re-acts upon
law, and change induces change, through an almost endless chain of
consequences; and it might be asked, why a simple law of matter should
thus be exempt from the common lot? Why, in a word, there should be no
intrinsic difference in matter, by which the gravitation of similar or
dissimilar substances should be affected? But experiment has detected no
such differences; a globe of lead and a globe of wood, of equal weight,
attract contiguous bodies with equal force. It is evident, therefore,
that if there be such differences, human means are not yet refined
enough to detect them. Was the issue successful then? Generally
speaking, we may say yes. But where there is a discrepancy between
theory and observation, however small that may be, it shows there is
still something wanting; and a high authority (Professor Bessel) says in
relation to this: "But I think that the certainty that the theory based
upon this law, _perfectly_ explains all the observations, is not
correctly inferred." We will not here enumerate the cases to which
suspicion might be directed, neither will we more than just allude to
the fact, that the Theory of Newton requires a vacuum, in order that the
planetary motions may be mathematically exact, and permanent in their
stability.