Hume argued that “reason alone can never be a motive to any action of the will.” Desire, for example, “arises not from reason.”

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Hume was most concerned with the nature of knowledge, morality, causality .

The Nature of Ideas

In this part, first David Hume divides all perception into ideas and impressions. He then argues that the simple impressions cause simple ideas, and from simple ideas form complex ideas, either restricted to the same order of the corresponding complex impressions (which are memories) or re-arranged in a new form (which is imagination). Descartes claimed that the only cause to the idea of God must be God himself, but according to Hume, God is a complex idea formed from simple ideas caused by simple impressions. Therefore, the idea of God neither requires God nor proves his existence.

Then Hume argues that general ideas are nothing but particular ideas attached to a certain word that gives it a wider application and makes it recall other individuals that are similar to it, for example we first see a particular man, then have an idea of this particular man, attach a word to this idea and then recall it when we see something similar (another man). Hume defends this view by 3 arguments - one of them is that the mind cannot think of a certain quality without the degree of that quality, such as a line without a length attached to it. Hence all ideas must have their particular degrees of qualities that therefore must be particular.

According to Hume it is through thinking of the resemblance of something with something else different in other aspects, for example we can consider the color of something only by thinking of the resemblance it has with something else of a different shape. Hume gives the example of a white marble globe and a black marble globe, one can think of the distinct shape by thinking of the resemblance between these two marble globes.

http://en.wikipedia.org/wiki/A_Treatise_of_Human_Nature#The_Nature_of_Ideas

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This book is a treatment of human cognition. It includes important statements of Skepticism and Hume's experimental method. Part 1 deals with the nature of ideas. Part 2 deals with the ideas of space and time. Part 3 deals with knowledge and probability. Part 4 deals with skeptical and other systems of philosophy, including a discussion of the soul and personal identity.

http://www.nytimes.com/2011/05/07/opinion/07zaretsky.html?_r=2&scp=2&sq=hume&st=Search