John Locke’s birthday, on August 29 in 1632, is not often celebrated or even remembered. This is a serious oversight. We should mark the day and honor the man for his enormously consequential contributions to the theory of knowledge and to political philosophy.

In 1689, Locke published his Essay concerning Human Understanding, an extended inquiry into what defines the limits of our ability to understand the objective reality of the world around us. Before Locke, it was widely assumed that the only significant limits on what we could understand about external reality were set by what there was to know about reality and that, at least in principle, we could over time learn more and more about reality until we knew everything there was to know.

Locke rejected this view of things. In the Essay, he argues that the limits to what we can know about external reality are defined by the limits of our mental faculties, i.e., the limits of our ability to use our senses to collect accurate data about the reality around us, and the limits of our ability to reason from these sensory inputs to form general ideas about the reality around us. Our faculties define the limits of our knowledge about reality, regardless of what happens to actually exist in the world around us.

Locke argued that, over time, we slowly build up our knowledge about the world around us by making observations, generalizing from our observations, and adjusting our generalizations as we make additional observations. We expand our understanding by this process of inductive reasoning.

Our observations may be incomplete or otherwise mistaken. Our generalizations may be faulty. Locke cautioned that we must, therefore, always be willing to modify our ideas in light of additional evidence. We must approach every question with an open mind, be tolerant of differing opinions, insist on the freedom of thought and speech essential for all learning, and always work to ensure that our ideas are firmly anchored in objective external reality.

Locke’s Essay qualifies him as a founding father of empiricism, which stimulated the rise of experimental science in the Enlightenment, and which still dominates philosophy throughout the English-speaking world.

Locke’s emphasis on tolerance connects the Essay directly to his political philosophy. Since human knowledge is based on limited sensory perception of the external world and is, therefore, neither certain nor complete, it would be both mistaken and morally wrong for political authorities to act intolerantly and impose their will upon the people.

In 1690, Locke set forth his political philosophy in detail when he published Two Treatises of Government. In the First Treatise, Locke attacks the concepts of divine right and absolute power for the monarchy. In the Second Treatise, he presents his alternative concept of government, one based upon a voluntary social contract formed among free individuals to establish a government for the sole legitimizing purpose of protecting their natural rights, their “Lives, Liberties and Estates.”

Locke stresses that the people retain their natural rights after the government they establish is up and running. If the government, once established, comes to abuse the natural rights retained by the people, or fails to effectively protect those rights, the people have a moral right—after seeking and failing to obtain redress through normal procedures—to overthrow the government and replace it with one better suited to achieve the legitimizing purpose.

It is clear from the briefest summary of the Two Treatises that Locke had a profoundly significant influence on the Founders. His political philosophy found clear expression in the Declaration of Independence and throughout the American Revolution. He was the political philosopher quoted most frequently by Americans in the 1770s. And Locke’s theories on empirical inquiry and intellectual tolerance shaped an openminded, commonsensical American approach to issues that helped secure unsurpassed progress and prosperity for our new Republic.

So on Monday the 29th, during a time when the principles and institutions of our Founding, tolerance, and the very concept of objective reality are all under attack, we should pause to mark well the day and truly honor John Locke: our Founding Philosopher.

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