The activity of the intellect is regarded in some quarters as something relatively superficial. Such a devaluation of the intellect is usually a reaction against an age of enlightenment and its one-sided overestimation of reason. What lends such a view some persuasive force is the fact that a certain kind of intellectual activity leaves the depths of the soul untouched. But this superficiality does not have its cause in the nature of the intellect. It is more correct to say that in such superficial intellectual activity the true power of the intellect is not fully unfolded. It may happen that two human beings listen jointly to the same news and that both have an intellectually clear grasp of its contents, such as, for example, the news of the Serbian regicide in the summer of 1914. However, the one "thinks no more about it," goes calmly on his way and a few minutes later is again busy with his plans for a summer vacation. The other is shaken in his innermost being....In his case the news has struck deeply at his inner life, and he understands the external events from the point of view of his own interiority. And because his full intellectual power is alive in his understanding, his mind penetrates into the context and into the "consequences" of the external event.
In this latter kind of thinking "the entire human being" is engaged, and this engagement expresses itself even in the external appearance. It affects the bodily organs, the heartbeat, and the rhythm of breathing, the individual's sleep and digestion. He "thinks with his heart," and his heart is the actual living center of his being. And even though the heart signifies the bodily organ to whose activity bodily life is tied, we have no difficulty in picturing the heart as the inner being of the soul, because it is evidently the heart that has the greatest share in the inner processes of the soul, and because it is in the heart that the interconnection between body and soul is most strikingly felt and experienced.
[From Finite and Eternal Being, quoted in Logos: A Journal of Catholic Thought and Culture 8.2 (2005) 183-193.]