Prof. Francisco Canals proposed that the metaphysics of knowledge of St. Thomas Aquinas, integrated by him in his work, could be expressed synthetically with the expression “thinking realism”. In this presentation I explain how from a realism of the act of being as perfection it can be found the place that corresponds to human knowing, as manifestive and as locutionary. In addition, sense is given to the dialogue that Canals established with Kantian criticism, by keeping in mind that a “thinking realism” which affirms knowing in its immateriality confronts the danger of idealism.