If it be a compound idea, it must arise from compound impressions.

If simple, from simple impressions.
I believe the most general and most popular explication of this matter, is to say [See Mr.Locke, chapter of power.], that finding from experience, that there are several new productions in matter, such as the motions and variations of body, and concluding that there must somewhere be a power capable of producing them, we arrive at last by this reasoning at the idea of power and efficacy.

But to be convinced that this explication is more popular than philosophical, we need but reflect on two very obvious principles.

First, That reason alone can never give rise to any original idea, and secondly, that reason, as distinguished from experience, can never make us conclude, that a cause or productive quality is absolutely requisite to every beginning of existence.

Both these considerations have been sufficiently explained: and therefore shall not at present be any farther insisted on.
I shall only infer from them, that since reason can never give rise to the idea of efficacy, that idea must be derived from experience, and from some particular instances of this efficacy, which make their passage into the mind by the common channels of sensation or reflection.
Ideas always represent their objects or impressions; and vice versa, there are some objects necessary to give rise to every idea.