Once we have the argument’s purpose or point clearly in mind and thus know what the arguer is aiming to establish, then we can look for the evidence, reasons, support — in short, for the grounds — on which that claim is based. In a deductive argument, these grounds are the premises from which the claim is deduced; in an inductive argument, the grounds are the evidence — which could be based on a sample, an observation, or an experiment — that makes the claim plausible or probable.
Not every kind of claim can be supported by every kind of ground, and, conversely, not every kind of ground gives equally good support for every kind of claim. Suppose, for instance, that I claim half the students in the classroom are women. I can establish the grounds for this claim in any of several ways. For example:
I can count all the women and all the men. Suppose the total equals fifty. If the number of women is twenty-five and the number of men is twenty-five, I have vindicated my claim.
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I can count a sample of ten students — perhaps the first ten to walk into the classroom — and find that in the sample five of the students are women. I thus have inductive — plausible but not conclusive — grounds for my claim.
I can point out that the students in the college divide equally into men and women and then claim that this class is a representative sample of the whole college.
Clearly, ground 1 is stronger than ground 2, and 2 is much stronger than 3.
Up to this point, we have merely restated points about premises and conclusions that were covered in Chapter 3. We want now to consider four additional features of arguments.