Tutorial #1. Conceivability & Possibility - A priori & A posteriori

Often in philosophy a priori methods are used to draw conclusions about what is possible and what is necessary.

Here is some excertps from Dave Chalmers ' Does Conceivability Entail Possibility?

"Arguments like this typically have three steps: first an epistemic claim (about what can be known or conceived), from there to a modal claim (about what is possible or necessary), and from there to a metaphysical claim (about the nature of things in the world)."

The method of conceivability.

"One argues that some state of affairs is conceivable, and from there one concludes that this state of affairs is possible. Here, the kind of possibility at issue is metaphysical possibility, as opposed to physical possibility, natural possibility, and other sorts of possibility. Metaphysical conclusions turn most directly on matters of metaphysical possibility: if one domain is reducible to another, the facts about the second should metaphysically necessitate the facts about the first. So it is metaphysical possibility that is relevant in the three-step argument above. And there is at least some plausibility in the idea that conceivability can act as a guide to metaphysical possibility. By contrast, it is very implausible that conceivability entails physical or natural possibility.

For example, it seems conceivable that an object could travel faster than a billion meters per second. This hypothesis is physically and naturally impossible, because it contradicts the laws of physics and the laws of nature. This case may be metaphysically possible, however, since there might well be metaphysically possible worlds with different laws. If we invoke an intuitive conception of a metaphysically possible world as a world that God might have created, if he had so chosen: it seems that God could have created a world in which an object traveled faster than a billion meters per second. So in this case, although conceivability does not mirror natural possibility, it may well mirror metaphysical possibility."

- What is meant by "conceivability" anyway?

In that paper, Chalmers distinguishes 8 senses...

In general,

Definition:
- conceivability is an epistemic notion (i.e. it has to do with a way of knowing).
It is the capability of "being conceived", of "being imagined". "What can be thought". It is NOT the ability to form mental images.
Chalmers characterizes it as "a property of statements, and the conceivability of a statement is in many cases relative to a speaker or thinker."

Example
You can conceive golden mountains.
You cannot conceive round squares.
You can conceive chiliagons, God, but presumably you cannot form a mental image of chiliagons or God.

- possibility is a metaphysical notion. It has to do with how things can be.
"Possibility" can also be understood in many different ways.
We have:
physical \ natural possibility.
metaphysical \logical possibility.

HERE You can find some very good lecture notes on possibility and conceivability by Jim Pryor


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The terms "a priori" and "a posteriori" are used in philosophy to distinguish between two different ways of knowing.
"A priori" and "A Posteriori" are also often used to distinguish different types of arguments.

Definition:
- a priori knowledge is independent of experience or experimentation. It is knowledge based on pure reason.

- a posteriori knowledge is dependent on experience or experimentation.

Examples

You know a priori that cubes have six sides; or that if today is Wednesday then today is not Thursday; that two plus five equals seven, etc.
In all these cases, it suffices that you understand the meaning of the relevant terms for you to gain that knowledge, you don't need to set up an experiment or to observe the world.

You can know a posteriori that some bachelors are beautiful; that Edinburgh is in Scotland; that water is H2O; that pain is correlated to he activation of certain brain areas.
In all these cases, it is not sufficient to sit comfortable in your armchair and reason about the meaning of the relevant terms for you to gain that knowledge. You have to go outside and observe how things are in the world.