Thomas Reid’s Common Sense Philosophy Part 2: On First Principles

The great conundrum faced by philosophers since time immemorial is the question “how do we know what we know?” This question falls under the subcategory of philosophy known as epistemology. Those who contend that all human knowledge arises through our senses are called empiricists. Those who believe that our knowledge is grounded in our ideas (i.e., our mental powers and state of mind) are often identified as idealists.

Enter the much better-known contemporary of Reid, the German philosopher Immanuel Kant (1722–1804), whose volume Critique of Pure Reason, was first published in 1781. What made Kant’s philosophy so important and ground-breaking was that Kant made a compelling case that while all knowledge begins with sense perception—the data received via our senses—the knowing process does not end there. Without the mind having the innate ability to act upon these sensations so as to transform them into knowledge, the world would remain unintelligible to us despite the data coming from our senses. Kant believed that there are specific mental categories which owe nothing to experience, and which are hard-wired into us which shape those sensations we do receive.

These so-called a priori categories (i.e., they are in place before we experience the world) include notions of time and space, logic, and mathematics. A creature without such categories would have the same sensations and see the same appearances we do, but would have an entirely different experience of them. My dog and I see the same tree. Since he does not have the mental categories I have, we experience the same tree quite differently. According to Kant, we see not the thing in itself, but only the thing as interpreted by our minds according to the a priori categories. This is reflected in Kant’s well-known distinction between the noumenal (the world beyond experience but which can reasonably be inferred from experience) and the phenomenal (the world which is actually experienced and accessed through the senses).

Reid, it has been said, worked backward from Hume’s skepticism to ask, “what would be necessary” if we are to know the world as it is? Reid wonders “what capacities must the human mind possess in order to truly know the external world?” He classifies these capacities as “first principles” which he believes are grounded in the so-called common sense of humanity. These principles are simply assumed and cannot be proven. We utilize them without any prior reflection upon them, nor can we prove them, because to do so we must utilize the very capabilities we are trying to prove. Although people universally reason from these first principles even if they do not believe in God, whenever we seek to get behind common sense to discover why things are the way they are, Reid argues there is no way to explain the existence of these first principles and the common sense of humanity apart from God who created the world and has designed us to live and act within that world. Reid writes,

I thank the Author of my being, who bestowed it upon me before the eyes of my reason were opened, and still bestows it upon me, to be my guide where reason leaves me in the dark. And now I yield to the direction of my senses, not from instinct only, but from confidence and trust in a faithful and beneficent Monitor, grounded upon the experience of his paternal care and goodness. In all this, I deal with the Author of my being, no otherwise than I thought it reasonable to deal with my parents and tutors. I believed by instinct whatever they told me, long before I had the idea of a lie, or thought of the possibility of their deceiving me.1

The theistic implications of Reid’s first principles ought not be overlooked. We reason via common sense because our Creator has designed us to do so.

Since Kant too was awakened through his interactions with Hume, we should not be surprised that both Reid and Kant were chasing the same goal. Reid wanted to challenge Hume’s skepticism, as did Kant—albeit Kant’s method of doing so was quite different than Reid’s. Reid appealed to those things assumed by all humans in all ages and across all cultures—identifying those first principles which ground human knowledge in the real world—and in not mere experience or in mental categories. This makes Reid a common sense realist—we do indeed apprehend the world as it is through ordinary daily activity. Reid really ought not be classified as a pure empiricist, although he does fit within the empiricist camp. Kant, on the other hand, thought the best way to do this was to define the limits of human reason by identifying those a priori mental categories which transform mere appearances of things into our knowledge of them. Kant’s so-called transcendental idealism sets out the premise that knowledge begins when we receive appearances of external things via sense perception, but we cannot regard these appearances as objects of knowledge until our minds organize these appearances through a set of fixed a priori categories already present in the mind.

The critical issue with which Reid and Kant were wrestling is that we all have to start the knowing process somewhere. But where, exactly? We must assume certain things to be true and already in place in our minds from our earliest years of self-consciousness and prior to experience of the world, otherwise our sensations would remain just that—mere sensations and never pass into knowledge.

Reid identified two types of first principles—necessary (certain) and contingent (probable) which provide the a priori framework necessary to understand the external world.2 Those first principles Reid identified as “necessary” (i.e., it is impossible to deny them) include logic (i.e., the law of non-contradiction) certain rules of grammar, mathematics, morals (i.e., unjust actions cause harm) and metaphysical realities—what we perceive actually exists. Reid is certain that God would never allow an evil demon to deceive us as Descartes once wondered. Reid also believed that whatever exists has a cause—in direct opposition to Hume’s skepticism about accepting things as true which he could not actually observe.

Those first principles which Reid identified as “contingent” include things such as consciousness of our own person (self-awareness), knowledge of the external world, that what I remember really did happen, that my personal identity truly exists as far back as I can remember, and that those things which I see and perceive really do exist. Reid also argued that we have the power as human agents to determine our own actions (we learn about causality through our own agentic powers, when as infants, we strike the mobile above our heads which causes it to move), that we are able to tell truth from error, we know that other minds exist, and that human testimony is ordinally true unless we have good reasons to believe otherwise. Reid added that the future course of the world will be similar to what it has been in the past.

Reid was clear as to the importance he placed upon such first principles.

All reasoning must be from first principles, and for first principles no other reason can be given but this, that, by the constitution of our nature, we are under a necessity of assenting to them. Such principles are part of our constitution, no less than the power of thinking: reason can neither make nor destroy them; nor can it do anything without them. . . . A mathematician cannot prove the truth of his axioms, nor can he prove anything unless he takes them for granted. We cannot prove the existence of our minds, nor even of our thoughts and sensations. A historian, or a witness, can prove nothing unless it be taken for granted that the memory and senses may be trusted.3

To deny these principles, Reid thinks, is absurd.

We must start the knowing process by assuming certain capacities are in place whether we can prove this or not. Reid begins with first principles, both necessary and contingent. Kant, on the other hand, rejects Reid’s realism, instead contending that we cannot see things as they really are, only our perceptions of them mediated through his famous “categories.”

Reid on Common Sense

For Reid, first principles and common sense are closely related. In Essays on the Intellectual Powers of Man, Reid writes, “first principles, principles of common sense, common notions, [or] self-evident truths” are “no sooner understood than they are believed. The judgment follows the apprehension of them necessarily, and both are equally the work of nature, and the result of our original powers.”[4] Previously, Reid identified common sense as “necessary to all men for their being and preservation, and therefore it is unconditionally given to all men by the Author of Nature.”5

More specifically, common sense refers to “the consent of ages and nations, of the learned and unlearned, [which] ought to have great authority with regard to first principles, where every man is a competent judge.”6 Such principles are identified as common sense because they are common to humanity and held by all people across time and cultures. Reid grounds his belief in common sense in an empirically justified generalization that this is the necessary state of affairs for humans to know anything—especially that the external world exists. To put it simply, this is common sense because it is demonstrably common to all of humanity. If all people see an object, and then universally assign the same qualities to that object, without any prior explanation or self-reflection required before doing so, this can only be because their knowledge of that object is true. All people instinctually think this way, unless convinced to doubt this knowledge by teachers of the ideal theory.

While Reid argued common sense was virtually self-evident because universal (in this regard, Reid is a foundationalist of a sort), his critics then and now, attacked him at this very point by claiming his common sense philosophy was nothing more than an appeal to majority opinion—the “wisdom of the vulgar.” If true, this chips away at the very idea of the supposed universality of Reid’s first principles. If common sense is really nothing but popular opinion verified by counting noses, and by observing how the uneducated rabble make decisions, then such first principles amount to nothing of value in settling truth claims. No philosopher worthy of the name would dare make such an appeal.

But Reid anticipated this line of criticism and as a good Newtonian, made clear that his first principles were actually empirical and psychological observations, reflecting the way people actually think and interact with the world around them. To give this point some teeth, at several places, Reid appeals to universal elements in the structure of human language (anticipating the later work of G. E. Moore and J. L. Austin).7 Reid points out that all human language is built upon a distinction between the active and passive voice, and that all languages distinguish between the qualities of things, and the things themselves. This goes a long way toward making Reid’s point.

To put it another way, Reid’s first principles are not true because most people accept them. Nor are they true because this is how the common man or woman expresses themselves when asked how they know what they know. Rather, people think and interact with the world as they do, precisely because this is how their creator has made them to think and act. Reid, to my knowledge does not refer to the divine image while discussing these common sense capabilities—although he does speak of the divine image in humanity when discussing moral liberty, and conscience.8 But are not the abilities given us by our creator something akin to humans reflecting the image of their creator? We are born with the capacity to utilize these first principles without any self-reflection, or without being able to give any reasons for doing so. This is how God made us and is his way of enabling and equipping his creatures to live in the world which he has made.


  1. Reid, Inquiry, in Works of, I.184.
  2. Reid, Intellectual Powers, in Works of, 1:435.
  3. Reid, Inquiry, in Works of, 1:130.
  4. Reid, Intellectual Powers, in Works of, 1:452.
  5. Reid, Intellectual Powers, in Works of, 1:412.
  6. Reid, Intellectual Powers, in Works of, 1:464.
  7. C. A. J. Coady, “Reid and the Social Operations of Mind,” in Terrence Cuneo and René Van Woudenberg, eds., The Cambridge Companion to Thomas Reid (New York: Cambridge University Press, 2004), 185.
  8. Reid, Active Powers of Man, in Works of, II.564, 585, 615.

©Kim Riddlebarger. All Rights Reserved.

You can find the whole series here.


Heidelberg Reformation Association
1637 E. Valley Parkway #391
Escondido CA 92027
The HRA is a 501(c)(3) non-profit organization

    Post authored by:

  • Kim Riddlebarger
    Author Image

    Kim is a graduate of Simon Greenleaf School of Law (M.A.), Westminster Seminary California (M.A., M. Div.), and Fuller Theological Seminary (Ph.D.). From 1995–2020 he was senior pastor of Christ Reformed Church (URCNA) in Anaheim. He was a long-time co-host of the White Horse Inn radio show and is currently Visiting Professor of Systematic Theology at Westminster Seminary California. Follow his work at The Riddleblog.

    More by Kim Riddlebarger ›

Subscribe to the Heidelblog today!

One comment

  1. I thought this was a very fine and fair explanation of Reid and distinction between Kant and Reid. Thank you.

Comments are closed.