Elements of the philosophy of the human mind. By Dugald Stewart. Rev. and abridged, with critical and explanatory notes, for the use of colleges and schools. By Francis Bowen ...

REASONING AND DEDUCTIVE EVIDENCE. 451
ing space are more nearly allied to geometrical theorems thlan we
might be disposed at first to apprehend; being involved or implied in the most simple and fundamental propositions which
occur in Euclid's Elements. When it is asserted, for example,
that "if one straight line falls on two other straight lines, so
as to make the two interior angles on the same side together
equal to two right angles, these two straight lines, though indefinitely produced, will never meet; " —is not the boundless immensity of' space tacitly assumed as a thing unquestionable?
assurance even that all snow is white; much less, that snow mast be
white."
The answer to this argument is really curious, as showing the writer's
incapacity of perceiving the distinction, which is a fundamental one, between necessary and contingent truths. " I cannot but wonder," says Mr.
Mill, " that so much stress should be laid upon the circumstance of inconceivableness, when there is such ample experience to show, that our capacity or incapacity of conceiving a thing has very little to do with the possibility of the thing in itself; but is in truth very much an affair of accident,
and depends upon the past history and habits of our own minds. There
is no more generally acknowledged fact in human nature, than the extreme
difficulty at first felt in conceiving any thing as possible, which is in contradiction to long established and familiar experience, or even to old and
familiar habits of thought." "There are remarkable instances of this
in the history of science; instances in which the wisest men rejected as
impossible, because inconceivable, things which their posterity, by earlier
practice and longer perseverance in the attempt, found it quite easy to
conceive, and which everybody now knows to be true." Mr. Mill proceeds to adduce, as such instances, the fact that there was a time when
men of the most cultivated intellects could not credit the existence of antipodes; could not conceive the force of gravity acting upwards; or that a
body could act upon the earth at the distance of the sun or moon.
The inference fiom this reasoning seems to be, that there is no proposition now regarded as a necessary truth, which may not, at some future
time, come to be generally disbelieved. In the future progress of knowledge, he seems to think it may be ascertained that the three angles of a
plane triangle are not equal to two right angles! It is certainly not impossible that the sun may not rise to-morrow; we can easily conceive that it
may not; though it is a fact attested by universal experience, that, to
every place in the torrid and temperate zones, the sun has risen once
in every twenty-four houras. Yet who does not perceive the difference,
in point of logical certainty, between the proposition that the sun will rise
to-morrow, and the axiom that two straight lines cannot inclose a space