What organs…does the power of the soul that we call ‘intellect’ make use of when it is active?…some locate it in the head, as though in a sort of acropolis…but it is located in the heart as in its own organ. And we know this because we are taught it not by men but by the Creator of man Himself when He says, “It is not that which goes into man’s mouth that defiles him, but what comes out of it” (Matthew 15:11), adding, “for thoughts come out of the heart” (Matthew 15:19).
St Makarios the Great says the same: “The heart rules over the whole human organism, and when grace takes possession of the pastures of the heart, it reigns over all a man’s thoughts and members. For the intellect and all the thoughts of the soul are located there.”
Our heart is, therefore, the shrine of the intelligence and the chief intellectual organ of the body. When, therefore, we strive to scrutinize and to amend our intelligence through rigorous watchfulness, how could we do this if we did not collect our intellect, outwardly dispersed through the senses, and bring it back within ourselves–back to the heart itself, the shrine of the thoughts?
It is for this reason that St Makarios–rightly called blessed–directly after what he says above, adds: “So it is there that we must look to see whether grace has inscribed the laws of the Spirit.” Where? In the ruling organ, in the throne of grace, where the intellect and all the thoughts of the soul reside, that is to say, in the heart. Do you see, then, how greatly necessary it is for those who have chosen a life of self-attentiveness and stillness to bring their intellect back and to enclose it within their body, and particularly within that innermost body within the body that we call the heart?
~St Gregory Palamas (Philokalia vol.4, p.334)