Argument Strength, the Toulmin Model, and Ampliative Probability

Abstract

Many within informal logic hold that to be logically good, the premises of an argument must constitute grounds adequate for the conclusion. This criterion is also expressed by saying that the premises must support the conclusion with sufficient strength or weight, sufficient that is to transfer the acceptability of the premises to the conclusion. But “strength,” “weight” are obviously metaphorical. What does argument strength or weight of premises mean literally? How in a given case may one determine degree of strength, and how much strength is necessary to be sufficient? These are open questions. To be sure, support is sufficient if the premises deductively entail the conclusion. To be sure also, for non-demonstrative arguments, many textbooks offer a “popular” answer – the premises must render the conclusion probable. More specifically, the probability of the conclusion, given the premises, must be sufficiently high.