Comp. 1Ti 4:4, 1Ti 4:5; Act 10:15; Mar 7:15, Mar 7:18, Mar 7:19; 1Co 10:26, 1Co 10:30; Rom 14:20. The aphorism is suggested by the commandments of men, Tit 1:14.
Unto them that are defiled (τοῖς μεμιαμμένοις)
Only here in Pastorals. See also Joh 18:28 (note); Heb 12:15; Jud 1:8. Only in Joh 18:28 in a ceremonial sense. Elsewhere of moral pollution.
Nothing is pure
Their moral pollution taints everything with its own quality. The purest things become suggestors and ministers of impurity.
Mind and conscience (ὁ νοῦς καὶ ἡ συνείδησις)
For νοῦς see on Rom 7:23 : for συνείδησις, see on 1Pe 3:16.