Descartes's Method of Doubt

Descartes thought that we could achieve absolute certainty by starting with radical doubt. He adopts this strategy in the Meditations on First Philosophy, where he raises sweeping doubts with the famous dream argument and the hypothesis of an evil demon. But why did Descartes think we should take these exaggerated doubts seriously? And if we do take them seriously, how did he think any of our beli...