Hume's quotation is from a famous passage discussing the "motivating influence of the will" in his Treatise on Human Nature and reads in full: Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them. (T 2.3.3 p. 415). read more
Reason is the slave of the passions in the sense that practical reason alone cannot give rise to moral motivation; it is altogether dependent on pre-existing desires that furnish motivational force. For Hume, this is not a fact we should lament (as moralists do) but a basic fact about our psychology. read more
No, reason is not a slave to the passions(emotions). Reason is a volitional process and is independent of the emotions. Furthermore, emotions are not tools of cognition. read more
Hume famously sets himself in opposition to most moral philosophers, ancient and modern, who talk of the combat of passion and reason, and who urge human beings to regulate their actions by reason and to grant it dominion over their contrary passions. read more