There are many ways to describe the single concept of probative weight, probative force, or probative strength. This concept is typically described with these types of physical metaphors. But which metaphor is the most isomorphic? This is an important question since it should determine which one we use in court.

Dr. Douglas Walton provides one perspective:

This analysis depends on what is meant by the expression ‘proving something’, in a sense that requires something more than just a valid (or structurally correct) argument. Such a notion of proving can be expressed in more precise terms by introducing the notion of probative weight. Probative weight is a concept of argument evaluation. The basic idea is that if premises have probative weight, and the argument from these premises to a conclusion is structurally correct, then the premises can throw probative weight onto the conclusion. An argument can be structurally correct if it is deductively valid, inductively strong, or if it fits the structure of a presumptive argumentation scheme. In such a case, the probative weight of the conclusion is increased as a function of two factors of the argument: (1) the probative weight of the premises, and (2) the probative weight (structural strength) of the argument from the premises to the conclusion. This type of case represents an increase in probative weight of a conclusion due to an argument supporting that conclusion…A probatively relevant argument can increase or decrease the probative weight of its conclusion…In a convergent argument, the conclusion needs to be revised upward to the value of the most plausible premise. In a linked argument, the probative weight of the conclusion needs to be revised upward to that of the least plausible premise. http://io.uwinnipeg.ca/~walton/papers%20in%20pdf/04fall_rel.pdf.

I have found this basic idea of an argument that has structural strength which goes from the premises to the conclusion effective in litigation. I respectfully suggest, however, that Dr. Walton’s use  of the term “probative weight” lacks sufficient precision as it is applied to the weight (i.e., heaviness) of the premises, to the structural strength of the argument, and to the substance that is transfered to the conclusion. I propose an alternative metaphoric concept that has more rigor and, I have found, is more effective in litigation to describe the nature of probative weight.

The probative “strength” of an inference bridge (optimally formed by essential predication transitivity) determines the amount (e.g., load) of certainty or acceptability, subjectively assessed by each juror, that each juror believes can reach the conclusion. The probative strength of an inference bridge is determined from the probative strength of each of the individual premises (both inference premises and the supporting assumption premises) which are part of a bridge. A structurally “correct” design of the inference bridge does not add to the strength of the premises. But a poor design can negate their strength. 

As is evident from the metaphor, the strength of any single inference bridge is only as strong as its weakest link (i.e., inference or assumption premise). Further, it becomes clear that multiple bridges that converge on the same conclusion can increase the amount of certainty that can reach it. And depending on their nature (e.g., undercutters, rebuttals) objections can be viewed as stresses that weaken an individual premise or as intervening or alternative bridges with their own strength that lead to different conclusions.