"You can trust that academics know the truth because they have tenure and are free to think for themselves."
I presume that statement was preceded by a certain claim made by an academic, which was then said to be credible simply because an academic said so. If this is the case, it is an appeal to authority. It generally goes as follows:
- A is an expert in domain D
- A claims C
- C is a question within domain D
- Therefore, C is true
Now, of course, it can be assumed that it is more likely that an expert (within his domain of expertise) is right than a random person. It just does not logically follow that this is the case. You have to evaluate the actual argument, not who made the claim.
Question: What percentage actually have tenure?
Quote from wikipedia:
The period since 1972 has seen a steady decline in the percentage of college and university teaching positions in the US that are either tenured or tenure-track. United States Department of Education statistics put the combined tenured/tenure-track rate at 56% for 1975, 46.8% for 1989, and 31.9% for 2005. That is to say, by the year 2005, 68.1% of US college teachers were neither tenured nor eligible for tenure; a full 48% of teachers that year were part-time employees.
So the claim that anyone in academia has tenure is simply false.
Does having tenure mean you're right about everything?
No, obviously not.
There are people with tenure who have conflicting views. Just to give a notorious example: Leonard Susskind and Stephen Hawking (who both have (or did have)) tenure have been in a fierce debate on information in black holes being completely lost or not. They disagree fundamentally, yet they are two of the world's top experts in their domain. And there are tons of examples of similar (yet not as famous) cases.