What is the definition of ability? More precisely, what is the definition of the relation "X is able to do Y"? For example, energy is defined as the ability to do work. Also, when a person becomes an adult, they are able to vote, even if they never actually will. I don't want a counterfactual definition that relies on the ontological extravagance of possible worlds, as I do not believe in the existence of possible worlds. I want a non-modal definition. Is there even such a rigorous, non-modal definition somewhere, in some text? Also, has any philosopher talked about this topic? I would like to see some references.
The concept of ability is closely related to that of a dispositional property. To say of common salt that it is soluble is to say it has the ability to dissolve in water. To say of paper that it is flammable is to say it has the ability to catch fire if you put a match to it. To say of alcohol that it is intoxicating is to say it has the ability to cause certain effects when consumed by animals. These are paradigm cases of dispositional properties.
In the context of human behaviour, ability often suggests an option to do something. I have the ability to swim, but I may choose never to do so. I have the ability to vote, but I am not obliged to.
Dispositional properties do not appear to work like categorical properties. They express a potency or a potentiality that may never be realised. To say that alcohol has the property of being intoxicating is a different kind of thing from saying that alcohol has the property of having a density of 0.789 kg/litre or a relative molecular mass of about 46.
There have been many attempts to provide an analysis of the concept, but no general concensus. Purely extensional accounts don't work. There are popular accounts based on hypothetical conditionals. There are causal accounts, including an influential one by David Lewis. Accounts such as these have to deal with apparent counterexamples in the form of masking or mimicking conditions and ceteris paribus conditions. Another popular line of analysis is to understand dispositions in terms of laws of nature and to regard them as essential properties that may or may not be actualised.
In most of these accounts, you won't be able to escape from talk of modalities and hypothetical conditionals. This does not commit you to the real existence of possible worlds.
There are a few key aspects that comprise a non-modal, rigorous definition of ability:
Capacity - Having the inherent attributes, skills, resources etc. needed to perform the action. e.g. a calculator has the computational capacity to perform arithmetic operations.
Opportunity - The circumstances allow for the performance of the action. e.g. an adult citizen has the legal opportunity to vote based on age.
Lack of impediments - Nothing is actively preventing or impeding the execution of the action. e.g. being physically restrained impedes movement.
Intentionality - For agents with a will, intentionally choosing to perform the action. e.g. an adult votes if they form the intent to do so.
Probability - A high likelihood the action would be successfully completed if attempted. e.g. a skilled pianist has a high probability of playing a simple composition correctly.
So a non-modal definition of "X is able to do Y" would require:
- X has the inherent capacity for Y.
- X has the situational opportunity to do Y.
- X faces no overriding impediments to doing Y.
- If X is an agent, X forms the intent to do Y.
- If X attempted Y, X would likely succeed.
This captures the key aspects of ability without referencing modal logic or possible worlds. Philosophers who have analyzed ability include Hume, Moore, and contemporary analytic philosophers like van Inwagen. The Stanford Encyclopedia of Philosophy has more on the analysis of ability.