Alex Sosler writes:

The rays of truth proceed from the sun, through Augustine’s head, to his heart, which he holds in his hand on the right side of the painting. This image is an apt illustration of Augustine’s thought. Truth doesn’t end in the head but makes its way to the control center, which is the heart. He understood the heart as being central to our living. What someone loves is more important than what they can consciously know or express. Truth is foundational but insufficient.

Sosler, A Short Guide to Spiritual Formation: Finding Life in Truth, Goodness, Beauty, and Community, p. 40

I’d nuance this a little differently. I’d say truth is foundational and indispensable, and for it to have full transformational effect, it must move beyond thought to affection. And, while we might read the painting from left to right, and thus understand the process as such, there is also a relationship running in the other direction, from right to left. It is vital, therefore, for both heart and mind to be fixed on truth. If we love the wrong things, our thinking becomes distorted. If we think the wrong things, our hearts become subject to damage and discouragement. But if heart and mind are compelled by what is true, together, the entire person is transformed.