In this book Maritain presents his view on how we know things, inspired by the ideas of St. Thomas Aquinas. According to him, to know something is to make its essence exist in an immaterial way in our mind. Maritain divides knowledge into two main types: rational knowledge, which includes science and philosophy, and suprarational knowledge, which encompasses faith and mysticism.
In the first part of the book, he talks about knowledge based on sensory experience, such as facts we can see or touch, and how we can derive universal laws from these facts. He also discusses metaphysical knowledge, which deals with realities beyond what we can directly experience. Additionally, he mentions three forms of wisdom: metaphysical, analogical, and theological.
In the second part, Maritain discusses the teachings of St. Augustine and St. Thomas, and how the highest wisdom comes from the Holy Spirit. He also examines the ideas of St. John of the Cross, who distinguishes between what can be explained with words and what is a profound experience that cannot be expressed.
Finally, Maritain explains that to know something, the subject (the person) and the object (what is known) must unite. In his view, knowing something is not just about learning, but also about changing in some way as a human being.