That is, these eternal and necessary differences of things make it fit and reasonable for creatures so to act; they cause it to be their duty, or lay an obligation on them so to do; even separate from the consideration of these Rules being the positive Will or Command of God, and also antecedent to any respect or regard, expectation or apprehension of any particular private and personal Advantage or Disadvantage, Reward or Punishment, either present or future, annexed either by natural consequence, or by positive appointment, to the practising or neglecting of these rules.

In the explication of this, nearly his whole system is contained.
His first concern is to impress the fact that there are necessary and eternal differences of ail things, and implied or consequent relations (proportions or disproportions) existing amongst them; and to bring under this general head the special case of differences of Persons (e.g., God and Man, Man and Fellow-man), for the sake of the implication that to different persons there belong peculiar _Fitnesses_ and _Unfitnesses_ of circumstances; or, which is the same thing, that there arises necessarily amongst them a suitableness or unsuitableness of certain manners of Behaviour.

The counter-proposition that he contends against is, that the relations among persons depend upon _positive constitution_ of some kind, instead of being founded unchangeably in _the nature and reason of things_.
Next he shows how, in the rational or intellectual recognition of naturally existent relations amongst things (he always means persons chiefly), there is contained an obligation.

When God, in his Omniscience and absolute freedom from error, is found determining his Will always according to this eternal reason of things, it is very unreasonable and blameworthy in the intelligent creatures whom he has made so far like himself, not to govern their actions by the same eternal rule of Reason, but to suffer themselves to depart from it through negligent _misunderstanding_ or wilful _passion_.

Herein lies obligation: a man _ought_ to act according to the Law of Reason, because he can as little refrain from _assenting_ to the reasonableness and fitness of guiding his actions by it, as refuse his assent to a geometrical demonstration when he understands the terms.