Summary
First Dialogue: Sensible qualities
Hylas accuses philosophers who deny material substance of scepticism. Philonous defines a sceptic as someone who denies sensible things and asks whether Hylas can explain how such things exist independently of perception.
They examine heat and cold. Intense heat is experienced as pain, and pain cannot exist without a mind. The same water can feel hot to one hand and cold to another, so temperature cannot be a single unperceived quality resembling both sensations.
Tastes, smells and sounds vary with bodily condition, distance and sensory organs. Colour changes under different light and appears differently to creatures with other visual capacities. Hylas concedes that these secondary qualities exist in perception.
He attempts to retain primary qualities such as extension, shape, motion, solidity and number. Philonous argues that these too vary with perspective and scale and cannot be conceived apart from sensory qualities. A visible shape is coloured; a tangible shape is felt.
Hylas then proposes that ideas represent material originals. Philonous objects that an idea can resemble only another idea and that a wholly unperceived object cannot meaningfully be compared with a perception.
The First Dialogue ends with material substance losing its sensible content. The objects directly perceived are ideas, and the proposal that they copy hidden material things has not been made intelligible.
Second Dialogue: Causes, spirits and God
Hylas tries to define matter as the substratum supporting qualities. When “support” is separated from ordinary spatial meaning, however, he cannot explain what the term signifies. An entirely unknown entity supplies no clear account of experience.
Matter is next proposed as a cause or occasion of ideas. Philonous argues that passive, inert matter cannot intelligibly produce sensory experience and that appealing to an unknown cause merely repeats ignorance in abstract language.
Philonous distinguishes ideas, which are passive and perceived, from spirits, which perceive and will. The self is not another sensory idea but an active subject known through its operations.
Sensory ideas arrive with a force, order and independence that distinguish them from voluntary imagination. Since finite minds do not create this system, Philonous attributes it to an infinite spirit.
God’s activity explains the consistency and persistence of nature. Laws of nature are the regular methods through which sensory ideas are produced, allowing finite minds to anticipate experience and act.
Hylas objects that this makes God the cause of painful or morally troubling events. Philonous separates natural causation from human moral agency: the predictable order is divine, while responsibility concerns how finite spirits will and use their powers.
Third Dialogue: Objections and common sense
Hylas worries that immaterialism destroys real objects. Philonous insists that houses, rivers, stones and trees remain real as sensible things. What is rejected is an imperceptible duplicate or substrate supposedly existing behind them.
Real ideas are distinguished from imagined ones by vividness, coherence, steadiness and independence from personal choice. Illusions remain mental events without having the ordered position in nature possessed by ordinary objects.
Statements about unperceived objects remain legitimate. To say that a table exists in an empty room means that it belongs to the stable divinely sustained order and would be perceived under suitable conditions.
Philonous denies that his position makes objects private possessions of individual minds. Different perceivers participate in an orderly public world because their experiences follow common laws established by God.
Scientific explanation also survives. Natural philosophers discover regular connections among ideas rather than hidden efficient powers in matter. Prediction and useful knowledge do not require belief in an unknowable substance.
Hylas raises questions about creation, scripture, bodily resurrection, other minds and the language of substance. Philonous argues that ordinary and religious discourse can be retained when interpreted without materialist abstraction.
The dialogue concludes with Hylas accepting that the very principles which appeared to lead towards scepticism return the thinker to common sense. Sensible things are precisely the realities immediately perceived, while material substance had placed reality permanently beyond experience.