921 has made us afraid of the term “Negative Corpus”. From what I understand the definition of Negative Corpus is the process of elimination to establish a cause where there is no hard evidence as to the source of ignition. Is it ever acceptable to hypothesize the source of ignition when the actual source is not found?

Everything you do in fire investigation is based on the process of elimination which is the definition of the negative corpus methodology. We observe something then we process that based upon our education, training, and experience. Then that is eliminated. By eliminating all that is not relevant we reach a conclusion as to what is relevant. Then we say that by using the scientific method to reach this finding that this must be the correct finding based on all the information that was present at the time the conclusion was reached. What is being said it what is described in Occam’s Razor. That is the idea that, in trying to understand something, getting unnecessary information out of the way is the fastest way to the truth or to the best explanation. What is being done when using the scientific method in testing hypotheses is one is getting unnecessary information out of the way. The lack of supporting evidence allows one to say that at the time this hypothesis was discarded there was insufficient evidence to support it as being valid. We cannot, based on science, say there will never be sufficient evidence to support the hypothesis.

One would have to ask about the fire investigation profession is it not the accepted task for the investigator to go into a structure that has hundreds of potential sources of ignition and eliminate them one by one until there is only one left to explain the ignition that took place. Yes, one wants there to be physical proof as to the actual source of ignition, but the question remains is it ever acceptable to hypothesize the ignition source without there being actual evidence, such as an item with some type of damage, that would show this item did supply the source of ignition. Is it not true that in most intentionally set fires the actual source of the ignition is hypothesized? Is the same not true when it comes to the ignition of fugitive gases?

When one states that by using the scientific method that have determined the most reliable explanation as to the cause of the incident. Is that not saying that there are other reliable explanations. Was it not the process of elimination being used that allowed the process to go from just being reliable to being most reliable?

Based on the comment that the scientific method requires evidence one must ask what the definition of evidence is as it is being used in this statement. If we are to communicate accurately, I suggest the used of definitions as instructed by the NFPA. For NFPA 821 it is the Marriam-Webster dictionary. This dictionary defines the word “evidence” as it is being used in this context as being something that furnishes proof.

All the examples given here earlier as to the formulation of a hypothesis was based on some type of evidence. I do not think that the formulation of a hypothesis that the hypothesis be tested at the time it is formulated. There may be insufficient information to conduct quality testing of that hypothesis at that time. Based on the comment about the most reliable hypothesis one is indicating that there was more than one hypothesis where the known evidence did not refute the hypothesis. What is being done is the use of elimination process to move from the category of reliable to most reliable.

Jim Mazerat
Forensic Investigations Group