ed, and natural changes cannot be brought about by mere
intentions. Others have said that light is the sun's substantial form,
but this also seems impossible for two reasons. First, because
substantial forms are not of themselves objects of the senses; for the
object of the intellect is what a thing is, as is said _De Anima_ iii,
text. 26: whereas light is visible of itself. In the second place,
because it is impossible that what is the substantial form of one
thing should be the accidental form of another; since substantial
forms of their very nature constitute species: wherefore the
substantial form always and everywhere accompanies the species. But
light is not the substantial form of air, for if it were, the air
would be destroyed when light is withdrawn. Hence it cannot be the
substantial form of the sun.

We must say, then, that as heat is an active quality consequent on the
substantial form of fire, so light is an active quality consequent on
the substantial form of the sun, or of another body that is of itself
luminous, if there is any such body. A proof of this is that the rays
of different stars produce different effects according to the diverse
natures of bodies.

Reply Obj. 1: Since quality is consequent upon substantial form, the
mode in which the subject receives a quality differs as the mode
differs in which a subject receives a substantial form. For when
matter receives its form perfectly, the qualities consequent upon the
form are firm and enduring; as when, for instance, water is converted
into fire. When, however, substantial form is received imperfectly,
so as to be, as it were, in process of being received, rather than
fully impressed, the consequent quality lasts for a time but is not
permanent; as may be seen when water which has been heated returns
in time to its natural state. But light is not produced by the
transmutation of matter, as though matter were in receipt of a
substantial form, and light were a certain inception of substantial
form. For this reason light disappears on the disappearance of its
active cause.

Reply Obj. 2: It is accidental to light not to have a contrary,
forasmuch as it is the natural quality of the first corporeal cause
of change, which is itself removed from contrariety.

Reply Obj. 3: As heat acts towards perfecting the form of fire, as an
instrumental cause, by virtue of the substantial form, so does light
act instrumentally, by virtue of the heavenly bodies,