Thomas Aquinas, one of the most influential medieval philosophers and theologians, discussed the concept of first principles as foundational truths that are self-evident and form the basis of all other knowledge and reasoning. These principles are considered to be universally true and are not derived from any other principles; instead, they are known intuitively or through the natural light of reason.

Key First Principles According to Aquinas. This is the foundation of common sense:

1. The Principle of Non-Contradiction:

Definition: This principle states that the same thing cannot both be and not be at the same time and in the same respect. For example, it is impossible for a statement to be both true and false simultaneously.

Significance: It is the most fundamental principle in logic and metaphysics, serving as the foundation for all rational thought and discourse.

2. The Principle of Identity:

Definition: This principle asserts that every being is identical to itself; in other words, “A is A.” A thing is what it is and not something else.

Significance: It establishes the basic understanding that things have a specific nature or essence, which is crucial for any further reasoning or identification.

3. The Principle of the Excluded Middle:

Definition: This principle holds that for any proposition, either that proposition is true, or its negation is true. There is no middle ground or third option between being and non-being.

Significance: It helps in determining the truth value of statements by eliminating ambiguity and ensuring clear, decisive reasoning.

4. The Principle of Sufficient Reason:

Definition: According to this principle, everything that exists has a reason or cause for its existence. Nothing can exist without a sufficient explanation for why it is as it is and not otherwise.

Significance: It underpins causal reasoning, which is essential for understanding the natural world, morality, and even the existence of God.

5. The Principle of Causality:

Definition: This principle states that every effect has a cause. Nothing comes into being without something else causing it.

Significance: It is fundamental to Aquinas’s arguments for the existence of God, particularly in his “Five Ways,” where he argues that the existence of contingent beings necessitates a First Cause, which is God.

For Aquinas, these first principles are not only logical but also metaphysical. They reflect the basic structure of reality and the way human reason is naturally inclined to understand the world. In his view, these principles are rooted in the nature of being itself and are accessible to all rational beings. Aquinas used these principles to build his philosophical and theological arguments, including his famous proofs for the existence of God and his understanding of natural law.