Meaning: The quote "It is possible to demonstrate God's existence, although not a priori, yet a posteriori from some work of His more surely known to us" is attributed to Thomas Aquinas, a prominent theologian and philosopher of the Middle Ages. Aquinas is best known for his synthesis of Christian theology with the philosophy of Aristotle, and his influential work in natural theology and metaphysics.

In this quote, Aquinas is expressing his belief in the possibility of demonstrating the existence of God through a posteriori, or "from the latter," rather than a priori, or "from the former," reasoning. A priori reasoning is based on deduction and logical reasoning, independent of experience, while a posteriori reasoning is based on empirical evidence and experience. Aquinas suggests that while it may not be possible to prove the existence of God through pure logic alone, it is possible to do so from some work of God that is more surely known to us through our experience of the world.

This idea is central to Aquinas's famous Five Ways, which are five arguments for the existence of God presented in his monumental work, the Summa Theologica. Each of the Five Ways is a form of a posteriori argument, drawing upon observations of the natural world and the order and causality found within it to demonstrate the existence of a transcendent, uncaused cause, which Aquinas identifies with God.

One of the most well-known of the Five Ways is the argument from motion, which begins with the observation that things in the world are in motion and asserts that there must be a First Mover, or God, who set all things in motion. Aquinas also presents arguments from causation, necessity and contingency, and degrees of perfection, all of which rely on the observation and experience of the natural world to demonstrate the existence of God.

Aquinas's emphasis on a posteriori reasoning in his natural theology reflects his conviction that the existence and nature of God can be known through the study of the created world. This approach aligns with his broader project of integrating faith and reason, seeking to demonstrate the compatibility of Christian belief with the insights of natural philosophy.

Furthermore, Aquinas's insistence on the use of empirical evidence and experience in demonstrating the existence of God has had a lasting impact on the development of philosophical theology and apologetics. His emphasis on the rationality of faith and the importance of engaging with the natural world continues to influence contemporary discussions of the relationship between science and religion, and the philosophical foundations of belief in God.

In conclusion, Thomas Aquinas's quote about the possibility of demonstrating God's existence a posteriori reflects his commitment to integrating empirical evidence and experience with philosophical reasoning in the pursuit of natural theology. His influential arguments for the existence of God based on observations of the natural world continue to be studied and engaged with by theologians, philosophers, and apologists, shaping ongoing discussions about the rationality of faith and the nature of the divine.