Rationalism 101: A Historical Overview

In the field of Epistemology, there are many positions and movements about how we acquire knowledge. Each of them proposes something about the ultimate source, criteria, and limits of our knowledge. However, out of all of them, rationalism—the view that reason is the primary basis of knowledge—seems to be the most popular one, as well as the oldest epistemological position. Some might even say that it’s as old as philosophy itself. That’s why, in this article, we’ll explore what the philosophy of rationalism really is, take a look at the philosophers that defended it, and provide an overview of its long-lasting tradition from antiquity till the modern day.

Before going into the history of rationalism, it’s necessary to explain what rationalism actually is. Roughly speaking, rationalism is a position in the theory of knowledge (epistemology). Supporters of rationalism claim that all human knowledge comes from reason, and the basic and fundamental means that we use in order to acquire knowledge and understand reality are based on higher intellectual capacities. Rationalist thinkers claim that reason is the ultimate source of all our knowledge, and that reason determines the criteria, abilities, and limits of our knowledge. Because of this, rationalists view logic and intellectual insight are the primary sources of knowledge, surpassing, for example, sensory experience and empirical observation.

Ancient Rationalism

Rationalism as a general philosophical theory and method of knowledge has a rich history in the Western philosophical tradition. From the ancient Pythagorean, Elean, and atomistic schools to Socrates, Plato, and Aristotle, countless thinkers have relied on rationalist methodology or approaches to solve philosophical problems.

Socrates’ rationalism was developed through his struggle with the sophists. His approach emphasized the use of dialogue to attain the clearest possible definitions of terms and gain an understanding of the truth and the virtues one should follow in life. According to Socrates, knowledge should be the driving force of all thinking and action. This concept is highlighted in his ethical-cognitive imperative: “Know thyself.”

However, Plato can be considered the true founder of ancient rationalism, as he was the first in ancient philosophy to develop a fully-formed system of knowledge, its sources, objects, criteria, possibilities, and scope. The basis for this system was Plato’s theory of ideas. Ideas are abstract, immaterial, perfect versions of actual things in the world. Plato believed that ideas are the root cause, principle, and foundation of all that exists; and that they exist in a separate world beyond the material, sensible world. Ideas “flow down” from the world of ideas to the sense-perceptible world, bringing the world into being.

The purpose of our existence, according to Plato, is to know and understand both the world of ideas, as well as the ideas themselves. This can only be achieved through the concept of “anamnesis” – that is, remembering the world of ideas and the knowledge we had until our soul was brought down to the realm of materiality and trapped in material human bodies. This remembrance is achieved through rational contemplation of the ideas. As such, Plato founded his epistemology on the principle of reason, making him the first thinker to develop an entire rationalist philosophy.

Aristotle, though he valued experience and its methods highly, was nonetheless one of the most important ancient rationalists. This is evidenced by his foundational works in the field of logic. In his Analytics, he established the theory of deductive reasoning – the method preferred by rationalists. Ancient rationalism reached its peak with the writings of Aristotle in his Metaphysics.

European rationalism reached its golden age in 17th-century philosophical thought with the philosophies of René Descartes, Baruch Spinoza, and Leibniz. They were rationalists in the fullest sense of the word, so rationalism is most closely associated with these 17th-century philosophers. Therefore, we will dedicate separate sections of this article to their work and ideas.

The French philosopher René Descartes (1596-1650) is considered the founder of the rationalist theory of knowledge. His contribution to the establishment and development of rationalism as a study of knowledge can be summarized in the following points:

1) First of all, for the absolute starting point of his philosophy, Descartes establishes the initial starting point: Cogito, ergo sum (I think, therefore I am). Therefore, he considers the abilities of the mind, primarily thinking, as the ultimate cognitive ability of a human being.

2) He considers reason to be endowed with innate ideas and principles, and because of that, he views it as the “natural light” of the mind. Thus, he establishes it as the most significant tool of knowledge and guarantor of truth.

3) In his work, “Meditations on First Philosophy,” Descartes advocates a critical examination of reason and formulates its rules.

4) Descartes also lays out the foundations of how we can gather evidence for a universal theory of truth. He posits that if something appears in a clear and distinct way to the mind, we can conclude that it is true.

5) Descartes tries to raise the basic method of knowledge to the level of a universal scientific method.

The Rationalist Philosophy of Baruch Spinoza

The second significant representative of rationalism, Baruch Spinoza (1632-1677), is characterized by the systematic implementation of the deductive method in his Ethics. Spinoza uses the method that Aristotle emphasized as the most important method of inference centuries prior.

In Ethics, Spinoza builds a comprehensive system that addresses metaphysics, epistemology, ethics, and political philosophy, all through the rationalist viewpoint.

The main point of his system is to demonstrate the inseparable connection of reason as the most significant cognitive power with the highest and most sublime goals of moral action in the sphere of human freedom.

To get into more detail, Spinoza posits a monistic view of reality, asserting that there is only one ultimate substance that encompasses everything in the universe.

This substance, which he called “God” or “Nature,” is infinite, self-caused, and comprises all existence. Everything that exists, including human beings and their thoughts, is a mode or manifestation of this fundamental substance.

In this metaphysical view of the world, Spinoza also embraced determinism, asserting that everything in the universe operates according to necessary causal laws. He argues that all events and actions, including human thoughts and behaviors, are determined by prior causes and cannot be otherwise; they occur necessarily.

Spinoza’s rationalist philosophy also extends to ethics. He aims to provide a framework for living a fulfilled and virtuous life and proposed that the highest virtue is attaining knowledge and understanding of the true nature of oneself and the universe.

For that purpose, Spinoza emphasized the importance of reason in guiding ethical behavior and advocated for a rational, joy-centered approach to life.

We can see that Spinoza attempted to apply the most basic principles of reason to every sphere of human life, and because of that, he is rightfully considered among the most important rationalist thinkers of his time.

Leibniz (1646-1716) can be considered the first real proponent of systematic rationalism in the realm of knowledge. He tackled nearly all of the questions of the theory of knowledge that had been discussed by his famous predecessors, such as Locke, Descartes, and Spinoza. Leibniz provided systematic, rationalistic responses to all inquiries regarding the origin, purpose, capabilities, limits, logical foundation, and values of human knowledge. As for all rationalists, and especially for Leibniz, the utilization of the method of pure deduction as a means of ascertaining the truth of our knowledge was of paramount importance.

Leibniz held that only reason could provide perceptions that are necessary truths. He asserted that all human knowledge is derived solely from reason or the soul, contending that reason is a source of illumination and clarity for the unclear and dim representations of the senses.

According to Leibniz, our knowledge is akin to two types of objects: objects of sense experience and objects of reason. Consequently, he developed his dichotomy of distinguishing between representations and truths of reason. Thus, he says that there are truths of facts (“les verites de fait”) and truths of reason (“les verites de raison”). Truths of facts are empirical judgments, and as such, they are contingent and insufficient to provide the foundation of science and philosophy. On the other hand, truths of reason are necessary truths, and as such, they have the most general or absolute validity. Such truths exist in logical laws and principles, as well as in mathematics and natural sciences, says Leibniz.

Contemporary philosophy recognizes the speculative philosophy of German classical idealist philosophers Hegel and Schelling as a form of rationalism due to their encyclopedic approach, and the high rationality associated with the speculative philosophical method. These philosophers sought to capture “the absolute” or absolute knowledge, with Hegel elevating this to the level of the identification between thought and being: believing all that exists is reason. According to him, the totality of the existence of the universe is simply the evolution and development of reason.

Marx (and subsequently, Marxists) are also attributed to a new type of rationalism: dialectical rationality. Even more advanced and modern variants of rationalism have developed since, such as Karl Popper’s critical rationalism.

Today, rationalism remains an important position in philosophy, influencing various fields such as epistemology, philosophy of science, and philosophy of mind. While the challenges to rationalism have spurred critical debates, the core principle of relying on logic and reason as a source of knowledge continues to shape our understanding of the world.