Subsection 7.3.5 We Omit the “Obvious”
But presuppositions aren’t the only things we leave out. We often construct arguments in which every statement has a truth value. But it isn’t possible to prove the conclusion using only the claims that have been explicitly mentioned.
[1] Jamie is holding Alex’s Ming vase.
[2] If Jamie lets go of the vase, Alex will be furious.
Probably none of us would question that [2] follows from [1]. But notice that, in evaluating the validity of argument, we bring to the table at least the following additional (unstated) premises:
[3] If Jamie lets go of the vase, it will fall to the floor. (Gravity)
[4] If a porcelain vase falls to the floor, it will break.
[5] A broken vase has very little value.
[6] An unbroken Ming vase has substantial value.
[7] People get furious when the value of their possessions is destroyed.
When we argue to each other, we rely on a pre-existing set of shared premises about our world.
If we want to apply our logical tools, we’ll have to be careful. We must make sure that our inference engine has all the premises it needs. We can do that either by providing them explicitly ourselves or by appealing to a knowledge base that has already been crafted by someone else.