So far, I have shown that we cannot rely on mere expert opinion if they speak outside of the topic that they know or if there is disagreement among experts on that question. But merely having agreement among the experts does not mean that we should believe their opinion. For all we know, their agreement is merely by chance. Therefore, their agreement is only useful if three further conditions are met. They must agree with the actual position, have some of the same reasons, and the reasons that they have must be from the same topic area that they are an expert in.
The first thing they must agree on is the actual position. Let’s suppose that we asked two philosophers whether or not free will existed. Suppose they both said that it did. Well, that is not quite good enough. The two philosophers may not mean the same thing when they say ‘free will’. One philosopher might mean that we are determined to do what we do, but as long as we do what we want we are free. Another philosopher might claim that we are not determined, and if we were, we would not be free. These philosophers do not agree on what free will is, so their agreement that we have it is not an agreement on the same position. This point is not limited to philosophers. Scientists have the same problems. What Einstein understood by ‘gravity’ and what Newton understood by ‘gravity’ are not the same either. So we cannot say that they both believed in gravity, since they did not believe in the same thing.
The second thing they must agree on is some of the reasons for the position. Furthermore, the experts must agree that these reasons are enough to support the position. Imagine that some scientists all believed that a theory was wrong, but none of them gave the same reason. Some said that the special tools didn’t work, others said that it violated previous evidence, and still others said that the theory was not a good explanation. The wrong theory was quantum mechanics. It is now accepted as a highly confirmed theory. So we should not believe in a position unless there is common agreement on the reasons for that position.
The third thing that the agreement must have is that the reasons for the position must be found within the expertise of the experts agreeing on the position. Consider the case of Cuvier. He was a French geologist who believed that there had been multiple catastrophes in geological history. The most recent one was supposed to be the Biblical Flood. His evidence for the Biblical Flood was found in the Bible, not in geology. Now Cuvier was a geologist, not a theologian. Therefore, he opinion on the flood, while shared with many Frenchmen of the time, was not something that should have been believed on his own authority.
This means that in order for an expert to convince you that a position is true, they must be an expert on the topic that includes that question, agree with 95% of other experts on the same actual topic, share enough reasons with all of these experts to believe the answer to that question and be an expert on the reasons for that question. Since I have covered both agreement and disagreement of experts, this is enough use expert arguments. But this does not mean that we should be convinced by all expert arguments. More on that next time.