From Bob Tostevin's book 'The Promethean Illusion: The Western Belief in Human Mastery of Nature':

After Poirot has picked up whatever clues he deems relevant, he likes to retire to an armchair or maybe to a table, where he may steady his nerves by building a house of cards: there and then, voila! He uses what he calls his "little grey cells" to order reality and penetrate the truth lying behind the appearance of things. Conan Doyle's famous detective Sherlock Holmes, on the other hand, proceeds from start to finish looking for visible clues that ultimately will lead him to the truth. Holmes' close observation of things is symbolized by the magnifying glass, and he will elicit from a person's garment, perhaps a hat, a complete picture of that person's age, socio-economic background, present financial status, psychological predispositions, etc. In contrast to Poirot's ratiocinative brilliance, Holmes' genius keeps always in direct observational touch with what the world offers him. Poirot is the French rationalist par excellence whereas Holmes is the English empiricist pure and simple.