We reason when we conclude one thing on the basis of something else. For example, one day you hear the sound of raindrops on the roof and conclude that it is raining outside. Logic as an academic subject is the theoretical study of reasoning. The ancient Greek philosopher Aristotle was the first person to write an actual treatise on the subject of logic, indeed he wrote five such treatises in all, and for this he is rightly called the Founder of Logic.

This contrast between reason and logic thus extends back to the time of Aristotle. Although the Greeks had no separate word for logic as opposed to language and reason, Aristotle's neologism "syllogism" (syllogismos) identified logic clearly for the first time as a distinct field of study.

No philosopher of any note has ever argued that logic is the same as reason. They are generally thought to be distinct, although logic is one important aspect of reason. But the tendency to a preference for "hard logic," or "solid logic," in modern times has incorrectly led to the two terms occasionally being seen as essentially synonymous (see Reasoning) or perhaps more often logic is seen as the defining and pure form of reason.

However machines and animals can unconsciously perform logical operations, and many animals (including humans) can unconsciously, associate different perceptions as causes and effects and then make decisions or even plans. Therefore, to have any distinct meaning at all, “reason” must be the type of thinking which links language, consciousness and logic, and at this time, only humans are known to combine these things.

Although this is an old discussion, the neurologist Terrence Deacon, following the tradition of Peirce, has recently given a useful new description in modern terms. Like many philosophers in the English traditions such as Hobbes, Locke and Hume, he starts by distinguishing the type of thinking which is most essential to human rational thinking as a type of associative thinking. Reason by his account therefore requires associating perceptions in a way which may be arbitrary (or nominal, conventional or "formal") - not just associating the image or "icon" of smoke and the image of fire, but, for example, the image of smoke and the English word "smoke", or indeed any made-up symbol (not necessarily a spoken word). What is essentially rational, or at least essentially human, is however not the arbitrariness of symbols, but how they are used. See below concerning Reason and Language.

Source : http://en.wikipedia.org