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As for his question, “Where in the body does the intellect reside?” the intellect subsists in the human soul, which reasons. As regards the body, it is connected to the heart, as Allah Most High says:

“Have they not travelled through the land, so that they may have hearts with which to reason?”

Ibn ‘Abbās was asked, “How did you acquire knowledge?” He replied, “With an inquisitive tongue and an understanding heart.”

However, the word “heart” may refer to the pine-cone-shaped piece of flesh on the left side of the body, which contains a black clot. As reported in the two Sahīh collections, the Prophet, may Allah bless him and grant him peace, said:

“Indeed, in the body there is a piece of flesh: when it is sound, the rest of the body is sound, and when it is corrupt, the rest of the body is corrupt.”

“Heart” may also refer to a person’s entire inner being, for the heart of something is its interior, like the kernel of a grain of wheat, an almond, a walnut, and the like. Hence a well is called a qalīb, because its heart—its interior—has been removed. Accordingly, when “heart” is used in this sense, the intellect is also connected to the brain. This is why it has been said that the intellect is in the brain, as many physicians maintain. This view has also been reported from Imam Ahmad. A group of his followers say, “The intellect originates in the heart; when it reaches maturity, it extends to the brain.”

The correct conclusion is that the spirit, which is the soul, is connected to both, and whatever it possesses