There is an inconvenience which attends all abstruse reasoning, that it may silence, without convincing an antagonist, and requires the same intense study to make us sensible of its force, that was at first requisite for its invention. When we leave our closet, and engage in the common affairs of life, its conclusions seem to vanish like the phantoms of the night on the appearance of the morning; and 'tis difficult for us to retain even that conviction which we had attained with difficulty. This is still more conspicuous in a long chain of reasoning, where we must preserve to the end the evidence of the first propositions, and where we often lose sight of all the most received maxims, either of philosophy or common life. I am not, however, without hopes, that the present system of philosophy will acquire new force as it advances; and that our reasonings concerning _morals_ will corroborate whatever has been said concerning the _understanding_ and the _passions_. Morality is a subject that interests us above all others; we fancy the peace of society to be at stake in every decision concerning it; and 'tis evident, that this concern must make our speculations appear more real and solid, than where the subject is in a great measure indifferent to us. What affects us, we conclude can never be a chimera; and, as our passion is engaged on the one side or the other, we naturally think that the question lies within human comprehension; which, in other cases of this nature, we are apt to entertain some doubt of. Without this advantage, I never should have ventured upon a third volume of such abstruse philosophy, in an age wherein the greatest part of men seem agreed to convert reading into an amusement, and to reject every thing that requires any considerable degree of attention to be comprehended. It has been observed, that nothing is ever present to the mind but its perceptions; and that all the actions of seeing, hearing, judging, loving, hating, and thinking, fall under this denomination. The mind can never exert itself in any action which we may not comprehend under the term of _perception_; and consequently that term is no less applicable to those judgments by which we distinguish moral good and evil, than to every other operation of the mind. To approve of one character, to condemn another, are only so many different perceptions. Now, as perceptions resolve themselves into two kinds, viz. _impressions_ and _ideas_, this distinction gives rise to a question, with which we shall open up our present inquiry concerning morals, _whether 'tis by means of our ideas or impressions we distinguish betwixt vice and virtue, and pronounce an action blameable or praiseworthy_? This will immediately cut off all loose discourses and declamations, and reduce us to something precise and exact on the present subject.