as the form of a house is caused to be in matter by art alone:
whereas other effects proceed sometimes from an exterior principle,
sometimes from an interior principle: thus health is caused in a sick
man, sometimes by an exterior principle, namely by the medical art,
sometimes by an interior principle as when a man is healed by the
force of nature. In these latter effects two things must be noticed.
First, that art in its work imitates nature for just as nature heals
a man by alteration, digestion, rejection of the matter that caused
the sickness, so does art. Secondly, we must remark that the exterior
principle, art, acts, not as principal agent, but as helping the
principal agent, but as helping the principal agent, which is the
interior principle, by strengthening it, and by furnishing it with
instruments and assistance, of which the interior principle makes use
in producing the effect. Thus the physician strengthens nature, and
employs food and medicine, of which nature makes use for the intended
end.

Now knowledge is acquired in man, both from an interior principle, as
is clear in one who procures knowledge by his own research; and from
an exterior principle, as is clear in one who learns (by
instruction). For in every man there is a certain principle of
knowledge, namely the light of the active intellect, through which
certain universal principles of all the sciences are naturally
understood as soon as proposed to the intellect. Now when anyone
applies these universal principles to certain particular things, the
memory or experience of which he acquires through the senses; then by
his own research advancing from the known to the unknown, he obtains
knowledge of what he knew not before. Wherefore anyone who teaches,
leads the disciple from things known by the latter, to the knowledge
of things previously unknown to him; according to what the
Philosopher says (Poster. i, 1): "All teaching and all learning
proceed from previous knowledge."

Now the master leads the disciple from things known to knowledge of
the unknown, in a twofold manner. Firstly, by proposing to him
certain helps or means of instruction, which his intellect can use
for the acquisition of science: for instance, he may put before him
certain less universal propositions, of which nevertheless the
disciple is able to judge from previous knowledge: or he may propose
to him some sensible examples, either by way of likeness or of
opposition, or some