Thomas Reid’s philosophy is a defence of common-sense principles against the scepticism that emerged during the early modern period of philosophy. For Reid, a belief counts as common sense if it is immediately believed upon being understood, cannot coherently be denied, and is required for ordinary human life and practical functioning (Nichols 2024, sec. 1.1). The belief in the existence and epistemic accessibility of external objects is one such principle. The common-sense beliefs that Reid identifies are not conclusions arrived at through inference; rather, he positions them instead as the necessary requirements for knowledge to be possible and treats them as the first principles of his inquiry. His strategy is not to provide indubitable justification for them but to undermine the rival position of representationalism and the way of ideas which he sees as positing unwarranted scepticism on common-sense. While some of his arguments against representationalism and the way of ideas are valid, exempting common-sense principles from the burden of proof cannot be justified. As a result, Reid’s strategy appears to beg the question against the sceptic by treating common-sense principles as epistemically authoritative without providing an independent justification.
The way of ideas refers to a theory of perception according to which the immediate objects of awareness are ideas or mental representations rather than external objects themselves (Nichols 2024, sec 3.1). In this context, the term ideas denotes mental contents generally, including both conceptual and perceptual states. The way of ideas posits that we never directly experience the world itself, only representations of it that present in mental states. The way of ideas is endorsed in some way by many of the great early modern figures including Descartes, Locke, Berkeley and Hume.
Reid’s strategy is to show how the way of ideas inevitably collapses into skepticism. If the way of ideas is true, then the immediate objects of perception are ideas. For knowledge to be possible, this means that perceiving an idea either (A) makes us aware of what it represents, (B) allows us to make an inference to the external world, or, (C) has no connection to an external world of objects because there is no distinction between ideas and objects.
For (A), when we perceive a representation brought about by a mental state, in order to claim we are aware of the object it corresponds to, there must be a justification that what we are experiencing necessarily represents the object as it exists. This justification cannot be given, only failed attempts according to Reid. The same problem of justification arises for (B), if we are directly aware of only our mental representations, making an inference from experiences to the external world goes beyond what experience permits. Both (A) and (B) appear to lead to scepticism. In both cases, there is no account for how it is possible to have knowledge of the external world. For Reid, this scepticism is self-undermining. If the mind has no access to an external world, it would not be possible to have any knowledge at all.
As to (C), Reid understands this as amounting to idealism. By definition, idealism is incompatible with the view that there are mind independent objects which Reid considers foundationally implausible. Because of its incompatibility with common-sense principles, Reid rejects (C).
Reid reassembles Newton’s first law of motion into a formulation that functions as a golden rule in his critique of the way of ideas: “No more causes, nor any other causes of natural effects ought to be admitted, but such as are both true, and are sufficient for explaining their appearances” (Nichols 2024, sec 1.2). If representationalism posits ideas as the intermediary between experience and external objects, Reid holds this is a violation of the law. He argues that ideas are posited unnecessarily and yield no explanatory gain. The only entities required in the explanation of the experience of an external object are the experience and the object itself. Adding the third element of ideas goes beyond what sufficiency demands.
Reid’s philosophy hinges on the privileged status of common-sense principles being immune to the burden of proof. Throughout history, there have been many examples of prima facie beliefs which, while widely considered to be common-sense, have eventually been overturned by newer ideas. The heliocentrism revolution and the discovery of evolution by natural selection can be counted as examples. The view that there are observable external objects has not been convincingly overthrown in a manner of the aforementioned examples; however, the possibility of this occurring at the hands of an ingenious new idea in the future cannot be overlooked. If this possibility can be admitted, then no privileged status should be given to Reid’s common-sense principles, and the burden of proof should be applied to them.
If the burden of proof is placed on the existence and epistemic availability of external objects, then the claim must face the entire epistemic process Reid attempts to undermine. Reid’s repurposed formulation of Newton’s first law would now work against the claim. In the case of explaining the experience of an external object, Reid would say the experience, and the object are sufficient elements for an explanation (Nichols 2024, sec. 3.1). Beyond minimal sufficiency, Newton’s law requires that any entities posited must be qualified as ‘true’. The existence of experience qualifies as true because it is self-evident and undoubtable. However, in the epistemic tradition of the early modern period, the existence of external objects is subject to unresolved scepticism. The uncertainty alone disqualifies the ‘true’ status of the existence of external objects, ultimately disqualifying them from being used as an element in an explanation according to Newton’s principle.
Reid’s common sense philosophy ultimately fails at the first layer. As long as the possibility for common-sense principles to be toppled by a better idea is open, there is no cause for exalting them with the status of being beyond the necessity of justification. Once pulled into doubt, they do not survive the rigor of early modern epistemology. Reid does offer a valid criticism of the way of ideas by showing that when the mind is taken to either become aware of, or infer, the external world, knowledge itself becomes impossible. While this critique is intended as an endorsement for common-sense, once common-sense is pulled into doubt it reads more like an argument showing that idealism is the only way representationalism can avoid scepticism.