A disposition is a property characterized by what it would manifestly produce under appropriate stimulus conditions—fragility manifests as breaking when struck, solubility as dissolving when placed in water. Analyzing dispositions requires specifying stimulus conditions (triggers), manifestation type (outcomes), and modal conditions (would, could, must). Debates concern whether dispositions require categorical bases and whether all causation is ultimately dispositional.
You already know from the categorical-dispositions distinction that some properties — like being fragile or soluble — are characterized by what they would do rather than what they currently are. Now the task is to analyze the internal structure of dispositions more carefully. Every disposition involves three elements: a stimulus condition (the trigger that activates the disposition), a manifestation (the outcome that results when the disposition is triggered), and, more subtly, a manifestation partner (the partner entity or condition that co-contributes to the manifestation).
The partner structure is easily missed but philosophically important. Fragility manifests as breaking when struck — but it takes two to break: the fragile glass and the striking force. The disposition is a property of the glass alone, but the manifestation requires cooperation from something outside it. This means dispositions are inherently relational: a disposition is always a disposition *to M when S*, where S is a stimulus condition that involves the world, not just the bearer. A key to acid will not dissolve steel; it needs an acid that acts as its partner. This relational structure has implications for how we understand causation — dispositions are not simply causes waiting to fire but properties poised within a network of possible interactions.
From your study of modal semantics and possible worlds, you can now sharpen what it means for a disposition to be unmanifested. A fragile glass that is never broken is still fragile — it is true in nearby possible worlds that, had it been struck, it would have broken. Dispositions are thus modal properties: they involve counterfactual conditionals. The analysis of a disposition is an analysis of which counterfactuals are true of its bearer. This is where the debate about categorical bases enters. Do dispositions float free as irreducible modal properties, or are they grounded in some categorical (non-modal) property — like the molecular structure of glass — that gives rise to the counterfactuals? Categoricalists say the counterfactuals are made true by an underlying categorical property; dispositionalists say some properties are irreducibly dispositional, and this is what grounds causation.
The practical upshot for ontology is significant. If all causation is ultimately dispositional — if every causal property is a power poised to produce a manifestation with its partners — then the world is less like a set of inert objects bumping into each other and more like a network of interacting powers constantly seeking their mutual manifestation partners. This view, called pandispositionalism, is now a serious position in contemporary metaphysics. Whether you find it compelling or not, understanding the stimulus-manifestation-partner structure is the analytical toolkit you need to evaluate it.
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