The starting point: the synderesis rule
The word synderesis is thought to be a corrupton of the Greek word syneidesis meaning 'knowledge within' or 'knowledge together'. In Aquinas' thought it is one of two words he uses for conscience, with the meaning "innate conscience".
Aquinas takes as his starting point a self-evident truth, that human beings by nature want to do good and avoid evil. The name given for this is the synderesis rule.
Notice this is something Aquinas doesn't prove as much as assume. Protestant authors like Niebuhr criticise Aquinas at this first step, because they argue that humans are by nature sinful, not good, and need to be redeemed by Christ. The argument demeans Christ, argues Niebuhr. In the United States the Manhattan Declaration by Evangelical Christians states : "the natural law forgets sin and thus depreciates the necessity of Christ and the supremacy of Scripture":
Aquinas calls synderesis a "natural habit".
"The first natural principles, given to us by nature, belong to a special habit, which we call "synderesis". So "synderesis" incites to good and murmurs at evil" (ST I Q79 A12).
Good, argues Aquinas, is the first thing practical reason grasps "since every agent acts for an end under some aspect of good" (ST 1-11 Q94 A2). Rational agents pursue the good naturally, because that is how God has designed us. This allows Aquinas to conclude that "whatever practical reason understands as man's good (or evil) belongs to the precepts of the natural law as something to be done or avoided".
Synderesis can also be seen as another word for conscience. Synderesis is the general, God-given, innate (inborn) tendency to pursue good ends. Of course, Aquinas acknowledges people sometimes fail to pursue good ends, but the underlying tendency is necessary if we are to admit that goodness is something natural to human beings as rational moral agents.