Glossary of terms (plus objections) for Descartes’ Meditations Meditation I epistemology

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Glossary of terms (plus objections) for Descartes’ Meditations
Meditation I


The theory of knowledge – what it is and how we can secure it


The idea that no knowledge is secure and that doubt is inevitable. In Descartes’ time people were sceptical about religious knowledge and scientific knowledge.

Methodological scepticism

To combat the sceptics, Descartes used their own method: doubting everything he could to try and discover knowledge which was secure.


The idea that knowledge must rest on a secure foundation. Mathematical knowledge is an example: it rests on axioms (self-evident, intuitively true propositions) and discovers new knowledge by employing self-evidently true operations (such as the Law of Equality where if you have an equation, adding (or subtracting etc.) anything to one side as well as the other maintains equality). Foundationalists hold that all knowledge must be like this.


The position that claims knowledge is primarily gained through thought.


The position that claims that knowledge is primarily gained through our senses. Descartes thought this position wrong (he is a rationalist) arguing that our senses are not totally reliable and so cannot deliver reliable knowledge.


A particular piece of knowledge delivered by the senses e.g. the red pen, the taste of coffee.

Dreaming Argument

Descartes’ puts an empiricist objection: surely I can know my own body exists? Well no, he answers, in dreams I appear to myself to be experiencing a ‘physical world’ but we know that there is not really such a physical world in dreams – so it is possible that I am now dreaming the ‘real waking world’ In that case, he lets the empiricist reply, even if the physical world does not really exist, that I am dreaming it exists, the fact that my ‘dream’ is crowded with objects implies (in the philosophical sense) that such objects must also exist somewhere outside my mind. Descartes replies that we cannot be so sure: in our dreams we concoct “sirens and satyrs” i.e. objects which are false. Finally, the empiricist can say that the sirens and satyrs are composed of components which cannot have been made up by my mind (the ‘Simples’), so implying they exist outside it.

Descartes’ Simples

The basic components which seem to be impossible to synthesise in the mind – including colour, extension, shape, mass, size, number, time.

Evil Demon Argument

His final strike against the empiricist and, indeed, against those thinking mathematical knowledge secure: even the ‘Simples’ might be false knowledge being fed into the mind by an evil demon (or ‘malign spirit’) together with a sense of certainty. Even though in dreams “2 + 3 = 5 and a quadrilateral figure has no more than 4 sides” the demon could vary these ‘truths’.

Psychological Certainty

He points out that certainty is not the same as truth. Mad people (who “claim to be kings”, “have an earthenware head”, “are made of glass”) are certain that they know ‘truths’ which we know to be false.


To foundationalism:

  • He gives no explanation or justification for the claim that all knowledge must be foundational in character. Scientific knowledge is as secure as we could wish but we know it is not foundational: it is based on theories which, by their nature, are provisional.

  • It is incoherent to jettison all beliefs as he recommends (like emptying all the apples from a basket and then replacing the sound ones). We need some beliefs to allow us to judge other beliefs – they can’t all be jettisoned.

To his rejection of empirical knowledge

  • False empirical knowledge is capable of being corrected (as Descartes himself says when he relates how a distant tower appears round but, closer to, turns out to be square) by ‘better’ empirical knowledge. The ‘counterfeit coins’ argument is an analogy: just as false coins are only possible if there are true ones around for comparison, false sense-data are only possible if there are true sense-data around for comparison. All empiricists have to do is ensure they are careful to work with the true ones rather than the false.

Descartes could reply to this latter objection by saying that he’s after total certainty (because he is a foundationalist rather than someone like today’s scientific society happy with the security of scientific knowledge) which cannot be delivered by sense-experience: just as even an expert cannot know for certain a particular coin is genuine, we cannot know for certain which are our ‘true’ sense-data.

Meditation II

Archimedean point

Fixed locus from which the Earth itself can be moved: secure foundation.

Cogito ergo sum

It is logically impossible to doubt one is thinking (since a doubt is itself a thought). This is proof against even the evil demon and provides a point of certainty: whenever I am thinking, I must exist.

Subjective knowledge

This argument establishes the primacy of the subject (the individual mind) in knowledge, i.e. rationalism is the route to true knowledge – so upsetting those who believe that objective (empirical) knowledge is primary.

Substance dualism

The belief that that there are two substances in the world: physical stuff and mental stuff.

Property dualism

The belief that there are two types of property in the world which are incompatible: mental properties such as sensations and feelings; physical properties such as light and sound.

Sum res cogitans

‘I am a thinking thing’: my essence is that of a thinking thing: without thought, I could not know that I exist. (Unlike my body which is not essential – I can exist without it, in theory.)

Leibniz Law

Two things are identical if they have the same properties. Descartes claims that since he cannot imagine his mind does not exist, but that he can imagine his body does not existence, they have different properties and hence are different things

The Wax Argument

On heating, the properties of wax which I get through my senses all change. It is the same wax because I understand it to be the same. Understanding is an attribute of the mind. This understanding comprehends the infinite (of shape, of size of the wax) which is impossible for the senses to do. Senses are linked to body/physical objects – hence this is evidence for dualism.


  • To know the Cogito, one must already know what are existence, thought and logical entailment, i.e. it isn’t foundational at all.

  • Knowledge is the exclusion of other possibilities – there is no meaningful alternative to ‘I know that I think therefore I am’. Hence, it is not knowledge.

  • It contains a suppressed premise (‘All thinking things exist’) which cannot be known.

Descartes could reply that it is not a syllogism – that when you notice the single example, this illustrates all examples (like in a maths proof).

  • He cannot know that there is an ‘I’ – all he could claim is that ‘there is thought’. He produces no evidence of the ‘self’ which has the mental attributes he claims for it.

  • Mental properties do not have to inhere in a ‘substance’.

  • He cannot argue for dualism using Leibniz’ Law since he employs the psychological verb ‘feign’ which is illegitimate.

  • He cannot know his mind with the certainty he claims: conscious thoughts are not sensibly definable within Descartes’ system.

  • The attributes of wax are not conceived of in isolation as Descartes’ analysis claims.

  • Descartes has not given a positive explanation of mind and so cannot claim it is better known than the body

Descartes could reply that the mind is unique and, as mental stuff, not open to the same sort of mechanistic explanations that physical stuff is open to.

Meditation III

Clear and Distinct Perception

A thought about which one could not possibly be wrong (such as the Cogito); the apprehension of this once the thought presents itself in the mind. Descartes uses it as his criterion for the truth.


A thought which has an object. This is in contrast to two other mental events: feelings and understanding. Ideas may be innate (carried within us from the start); adventitious (impressed on the mind from outside); factitious (synthesised in the mind).

Objective reality

The notion that our mental events are real, but that they have different degrees of reality. Descartes thinks that no particular mental event can have any more objective reality than the mental event which caused it to arise.

Trademark (cosmological) argument

Given the notion of objective reality, and allowing that God is the superior mental event that a mind can have, God the idea cannot have been synthesised by anything less than God. Since Descartes has the idea of God, God must exist.

Cartesian Circle

Descartes’ critics say that his argument is circular and hence empty: ‘I am certain God exists because I see it clearly and distinctly. I can be certain of what I see clearly and distinctly because God exists.’

Epistemological necessity

For knowledge (beyond the bare Cogito) to be a possibility, God’s existence is necessary. (Otherwise, scepticism via the evil demon possibility is always present.)


  • ‘truth consists in clear and distinct perception’ is hardly as plainly true as a self-evident truth like 1 = 1 and so is not useful as a criterion for the truth. Also, ‘self-evidence’ seems to vary from individual to individual; such truths are not always immediately clear.

  • It is rational to believe truths that are not self-evident so his criterion is far too narrow.

  • In the Trademark argument the premise about greater objective reality of the ‘causing’ thought relative to the ‘resulting’ thought is suspect: e.g. is the thought of solidity caused by something more solid than the thought? Seems highly doubtful.

  • ‘Perfection’ as an idea can be arrived at more naturally by extrapolation from our experience (e.g. such as a perfectly straight line).

  • All cosmological arguments rely on the knowledge that ‘existence cannot appear from nothing’ being already being in place before the argument can take place (such as the ‘objective reality’ premise). This is not knowable a priori (and Descartes hasn’t shown how he knows it) and so cannot be used to produce the existence of God.

Descartes’ reply to the criticism of the circular argument accusation hinges on memory: the evil demon can tamper with remembered thoughts, but not thoughts that are being actively entertained at present. Current clear and distinct thought makes memory (and hence the evil demon) to that thought (such as God’s existence) irrelevant.
Meditation IV

Good God

Deceit is an imperfection and so could not be a part of the perfect God he knows.


The ability to apprehend things clearly and distinctly. Since it can deliver the truth, it is a perfect attribute. However, unlike God’s understanding, it is finite (Descartes knows he doesn’t know everything).


The mental faculty for judging between things. This capacity can range unbounded over all the possibilities that a mind can engender: it is infinite in its scope.


Though possessing perfect gifts from perfect God (will and understanding), mistakes can arise from judging outside the sphere of one’s understanding.


  • Understanding is not immediate and universal – people vary in the degree to which they ‘understand’ things. Without a sharp defining line, his appeal to not judging outside the sphere of understanding is impossible.

  • It is impossible to withhold judgment in the way Descartes insists we must – radically, we must judge whether we understand something clearly and distinctly in the first place (which Descartes says we must do before we judge…)

Meditation V

Res extensa

The ideas of material objects always carry with them attributes such as duration, size and shape. Without the notion of dimension (of space/time), such ideas would be impossible. Hence, the essence of the ideas of material things is extension.

Geometric truths

Truths about perfect triangles are possible despite not being prompted by anything external to Descartes’ mind. In other words, truth can be derived from a concept.

Ontological argument

‘I have the concept of a supremely perfect being. Existence makes something more perfect than non-existence. Hence it is contradictory to say that a perfect being does not exist (and so a perfect being does exist.’


  • Knowledge of ideas of physical objects must be a priori at this stage of Descartes’ argument. He provides no justification for their being knowable in this way.

  • Defining something into existence is illegitimate (‘a T-fairy is a fairy that truly exists. It is now contradictory to say ‘a T-fairy does not exist’ therefore T-fairies really do exist’ is plainly absurd.

Descartes replies that God is not like T-fairies. What makes God special is that he is a unity that includes all the perfections. T-fairies and the like do not and hence for them this proof is unavailable.

  • The proof is illegitimate because it rests on a comparison of the imagined existence and real existence – he has not shown that the mental world and the real world are one and the same thing.

Meditation VI


The process of the mind which leads to understanding.


Descartes’ term for mentally visualising some object or other.

Chiliagon argument

A thousand-sided figure is impossible to imagine and yet which can easily be grasped by intellection. This demonstrates the difference between imagination and intellection: imagination requires effort unlike intellection.

Natural light (or ‘by the light of nature’)

An alternative for clear and distinct perception (and which emphasises the nature that God has made).

Imagination as an attribute of body

Descartes knows that extension is not essential to the mind. He also knows that extension is essential to imagined objects (from Meditation V). Thus, imagined objects cannot arise from the mind and so do arise from elsewhere: body.

Existence of physical objects

It is possible that these imagined objects are not real but this would involve God in a deception. Since God is not a deceiver, physical objects (which give rise to the imagined objects) exist.

Dualism argument

In a nutshell: it is possible for God to bring about any A and any B as distinct entities if Descartes can perceive them as clearly and distinctly different. He clearly and distinctly perceives mind and body as essentially different. Hence mind and body are distinct entities.

Platonic dualism argument

The body is divisible; the mind is indivisible. This shows that they are separate things.

Dualism and error

Given that God cannot make material things that are unextended, it is possible to introduce error into the body (such as stimulating a nerve anywhere along its length and the brain recording it as a stimulus applied at the – normal – end). This is not God’s fault – especially as he has given us senses for checking such oddities and understanding to find the truth.

Reliability of the senses

It is natural to trust one’s senses and, since it is God who created nature and Descartes, the information from the senses can generally be trusted. God has done what he can to make them reliable.


Knowledge of one’s past experiences and thoughts may be used to help filter information from the senses to arrive at the truth.


  • It is eminently possible for the mind to be the brain in operation. To prove otherwise, Descartes must show that the mind operates in a different way from any explanation of how the brain operates. This he fails to do. Indeed, the mind and/or brain develop or deteriorate in step (‘instant idiot: just add alcohol’) – an observation making dualism superfluous.

  • Though his chiliagon example is a good one, it is exceptional: there is not the same clear-cut distinction between what can be thought and what can be imagined.

  • His dualism argument only shows that mind-body separation are possible rather than actual.

In reply, Descartes would say that this is the point: they are separable (and hence distinct) even if it is only as a possibility.

  • His dualism argument rests on the reliability of his ‘clear and distinct perception’ which is suspect (see above).

  • It is possible that Descartes is not just a thinking thing (as per Meditation II), his physical body may also be included in that essence. In other words, his perception could be incomplete and there is no way that Descartes has shown to demonstrate the limits of his clear and distinct perception lies solely with the mind.

  • As in Meditation II, Descartes provides no evidence that there is one mind per body – and hence knowledge of the ‘self’ as commingled mind and body is not possible.

  • Descartes offers no coherent account of how the mind and the body could possibly interact.

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