an into sin. In this sense it is said to be
his proper office to tempt: for thought at times man tempts thus, he
does this as minister of the devil. God is said to tempt that He may
know, in the same sense as that is said to know which makes others to
know. Hence it is written (Deut. 13:3): "The Lord your God trieth
you, that it may appear whether you love him."

The flesh and the world are said to tempt as the instruments or
matter of temptations; inasmuch as one can know what sort of man
someone is, according as he follows or resists the desires of the
flesh, and according as he despises worldly advantages and adversity:
of which things the devil also makes use in tempting.

Thus the reply to the first objection is clear.

Reply Obj. 2: The demons know what happens outwardly among men; but
the inward disposition of man God alone knows, Who is the "weigher of
spirits" (Prov. 16:2). It is this disposition that makes man more
prone to one vice than to another: hence the devil tempts, in order
to explore this inward disposition of man, so that he may tempt him
to that vice to which he is most prone.

Reply Obj. 3: Although a demon cannot change the will, yet, as stated
above (Q. 111, A. 3), he can change the inferior powers of man, in a
certain degree: by which powers, though the will cannot be forced, it
can nevertheless be inclined.
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THIRD ARTICLE [I, Q. 114, Art. 3]

Whether All Sins Are Due to the Temptation of the Devil?

Objection 1: It would seem that all sins are due to the temptation of
the devil. For Dionysius says (Div. Nom. iv) that "the multitude of
demons is the cause of all evils, both to themselves and to others."
And Damascene says (De Fide Orth. ii, 4) that "all malice and all
uncleanness have been devised by the devil."

Obj. 2: Further, of every sinner can be said what the Lord said of
the Jews (John 8:44): "You are of your father the devil." But this
was in as far as they sinned through the devil's instigation.
Therefore every sin is due to the devil's instigation.

Obj. 3: Further, as angels are deputed to guard men, so demons are
deputed to assail men. But every good thing we do is due to the
suggestion of the good angels: because the Divine gifts are borne
to us by the angels. Therefore all the evil we do, is due to the
instigation of the devil.

_On the contrary,_ It is written (De Eccl. Dogmat. xlix): "Not all
our evil thoughts are stirred up by the devil, but