Professor Tillers recently posted a very interesting and informative Rationale argument map for an inference problem in United States v. Robinson, 560 F.2d 507 (2d Cir., 1977) (en banc). http://tillerstillers.blogspot.com/2007/05/tillers-tries-to-be-rationale.html. It is a wonderful illustration of the complexity of witness credibility attributes.

In Rationale software, these attributes, as shown by Professor Tillers, fit functionally alongside the inferential premises. As discussed in an earlier post here, some of the attributes (e.g. the witness was in a position to see and hear the perpetrator) are necessary “conditions” for one to draw an inference from the testimony. Other attributes (e.g. the witness was unbiased) are merely “companions” to a witness argument scheme since they only affect the strength of the inference. For example, the testimony of a biased witness could still have some probative force.

While certainly not always necessary, when a further purpose of an argument map is to be sure to reveal any possible hidden premises (other than “conditions” and “companions” for which an infinite are possible) in the inference steps, the complete Path argument schema ensures this goal. The Path schema provides a rigorous, but simple scaffolding for the designer. It consists of simply joining the subject of the ultimate probandum to its predicate through a series of interlocking sentences in which the predicate of one sentence is the subject of the following sentence. This creates a transitive inference path for the subject of the ultimate probandum to reach its predicate through a series of inference steps. Since each sentence consists of interlocked “co-premises” there is no space for a hidden premise to exist. Note, however, that this claim is not the same as saying that further granulation would not be possible. An infinite number of intervening co-premises could always be imagined. Ensuring no hidden premises simply means that there are no missing co-premises that would be needed for the chain of inference since each co-premise is already interlocked with the next.

The following is a Rationale Path map illustrating these interlocking co-premises for the same inference problem in Robinson. (The issue of witness credibility has been simplified for this different purpose.) This Path map reveals the two co-premises hidden in the Tillers map. These two co-premises are the major flaws in the argument made by the prosecution. As shown below, the first weak co-premise is that “actual possession of a .38 caliber revolver on July 25, 1975 indicates that one possessed a .38 caliber revolver on May 16, 1975 (ten weeks earlier).” The other weak co-premise is that “possessing a .38 caliber revolver on May 16, 1975 (the date of the robbery) indicates that one possesses the actual revolver that was used by the perpetrator at the robbery.” With these co-premises revealed in the inference chain, the attorney can more effectively argue why the inference chain should break at these two links. Or in the alternative, the attorney can effectively argue that the small probative force is easily outweighed by the prejudicial effect.