Why accept a premise?

1. Defended in a subargument that is cogent.

Necessary truth (impossible to be false)

Common knowledge: acceptable to virtually everyone.


Proper authority - pro-homine (appeal to authority) and ad-hominem (attack on the person).

A good pro-homine argument: one which argues that a claim should be accepted because it is held by someone who we regard as knowledgable, trustworthy, and free of bias.

Appeal to Authority: Authority doesn't mean
position, but expertise.

Rules for good appeals to authority:

  1. Authority must be identified. 9 out of 10 doctors doesn't tell us much.
  2. The authority must not be biased. Some people will say anything for money.
  3. If there is no consensus among authorities, this must be mentioned.
  4. The issue being addressed must belong to the authority's body of knowledge. (Ph.D.'s don't know everything) ex #1
  5. The appeal must be one in which there is knowledge to be had. facts vs. evaluation. Some highly abstract areas are not ones in which expert opinion is available.


An ad hominem argument is an attack against the person. A good ad hominem attacks an issue that is relevant. It is a premise about the background, personality, or character of a person. It is irrelevant to the merits of his theories and arguments, except in the case that the theories and arguments are about himself. Specific points about a person's background may bear upon the reliability of his testimony or the legitimacy of his authority. That means that they are relevant to our decision whether to accept his claims on his testimony, even though they are not directly relevant to the question of whether these claims are true or false. The criteria are that a person's views should be rejected because they are deemed i) not knowledgable, ii) untrustworthy, or iii) biased. ex #2

When could an ad hominem argument be acceptable?

accepting premises "for the sake of argument."

Why reject a premise?

1. Premise is false.

Two premises will explicitly or implicitly contradict one another. (opposite of necessary truth)

Dependence on faulty assumptions. Hidden assumptions that are either debatable or false.

Premises is not more certain than the conclusion. Purpose of this sort of argument is to lead from more to less certain claims.