The fourth part of Professor Dan Robinson's series examining Reid's critique of David Hume. “It is evident that a power is a quality, and therefore can’t exist without a subject to which it belongs…This (Humean) suggestion— There exists some power that cannot be attributed to any thing, any subject, which has the power —is an absurdity…No principle seems to have been more universally acknowledged by mankind ever since the first dawn of reason than that every change we observe in nature must have a cause…Another argument to show that all men have a notion or idea of active power is that there are many mental operations—performed by everyone who has a mind, and necessary in the ordinary conduct of life—which presuppose that we have active power”.