Rationalism is a philosophical epistemology emphasizing reason as the primary source and ultimate test of knowledge, often to the exclusion of, or in preference to, sensory experience. Its basic tenets include:

1. Primacy of Reason: True knowledge is derived from intellectual deduction, not empirical observation.

2. Innate Ideas: Certain ideas or principles are inherent in the mind from birth, not acquired through experience.

3. Clear and Distinct Perception: Knowledge is attained through ideas that are so clear and distinct they cannot be doubted.

4. Mathematical Method: Logic and mathematical deduction are the ideal models for philosophical inquiry, aiming for certainty.

René Descartes, a founding figure of modern rationalism, meticulously built his philosophical system in direct consonance with these tenets.
He began with methodological doubt, systematically doubting all sensory experience to find an indubitable truth. This led to his foundational statement, "Cogito, ergo sum" ("I think, therefore I am"). For Descartes, the existence of the thinking self was an absolutely clear and distinct perception, apprehended purely by reason, not sensation.

From this indubitable starting point, Descartes proceeded to deduce further truths, much like geometry. He argued that the idea of God as a perfect being, and the ideas of mind (thinking substance) and matter (extended substance), are innate ideas—placed in us by God, rather than acquired empirically. He then deductively reasoned God's existence and, based on God's non-deceiving nature, the reliability of clear and distinct perceptions and, cautiously, the external world. His dualism between mind and body, and his mechanistic view of the physical world, were all derived through rational deduction from these innate, clear, and distinct ideas, mirroring the certainty and structure of the mathematical method. Thus, Descartes' philosophy is a quintessential example of constructing knowledge purely from the bedrock of reason.