All the perceptions of the mind are of two kinds, viz. impressions and ideas, which differ from each other only in their different degrees of force and vivacity.  Our ideas are copyed from our impressions, and represent them in all their parts.  When you would any way vary the idea of a particular object, you can only encrease or diminish its force and vivacity.  If you make any other change on it, it represents a different object or impression.  The case is the same as in colours.  A particular shade of any colour may acquire a new degree of liveliness or brightness without any other variation.  But when you produce any other variation, it is no longer the same shade or colour.  So that as belief does nothing but vary the manner, in which we conceive any object, it can only bestow on our ideas an additional force and vivacity.  An opinion, therefore, or belief may be most accurately defined, a lively idea related to or associated with a present impression.

We may here take occasion to observe a very remarkable error, which being frequently inculcated in the schools, has become a kind of establishd maxim, and is universally received by all logicians.  This error consists in the vulgar division of the acts of the understanding, into conception, judgment and reasoning, and in the definitions we give of them.  Conception is defind to be the simple survey of one or more ideas:  Judgment to be the separating or uniting of different ideas:  Reasoning to be the separating or uniting of different ideas by the interposition of others, which show the relation they bear to each other.  But these distinctions and definitions are faulty in very considerable articles.  For first, it is far from being true, that in every judgment, which we form, we unite two different ideas; since in that proposition, god is, or indeed any other, which regards existence, the idea of existence is no distinct idea, which we unite with that of the object, and which is capable of forming a compound idea by the union.  Secondly, As we can thus form a proposition, which contains only one idea, so we may exert our reason without employing more than two ideas, and without having recourse to a third to serve as a medium betwixt them.  We infer a cause immediately from its effect; and this inference is not only a true species of reasoning, but the strongest of all others, and more convincing than when we interpose another idea to connect the two extremes.  What we may in general affirm concerning these three acts of the understanding is, that taking them in a proper light, they all resolve themselves into the first, and are nothing but particular ways of conceiving our objects.  Whether we consider a single object, or several; whether we dwell on these objects, or run from them to others; and in whatever form or order we survey them, the act of the mind exceeds not a simple conception; and the only remarkable difference, which occurs on this occasion, is, when we join belief to the conception, and are persuaded of the truth of what we conceive.  This act of the mind has never yet been explaind by any philosopher; and therefore I am at liberty to propose my hypothesis concerning it; which is, that it is only a strong and steady conception of any idea, and such as approaches in some measure to an immediate impression. [Footnote 5.]