[PL 211] Is the independent rational person a myth?

In Logical Self-Defense, Johnson and Blair (1994, pp. 167-168) write:

it would be a mistake to picture the autonomous self-guiding, belief-scrutinizing individual as a person who sets off completely independently, an isolated rational agent building up a system of beliefs by carefully checking and confirming each "belief" candidate before adding it to the store of trusted claims or discarding it as unverified or false. For one thing, the very classification and testing systems such a person would have to use would themselves at some point just have to be accepted, at least for the time being; otherwise there would be an infinite regress of testing tests and testing concepts. For another, it is simply physically and mentally impossible for one person to verify everything. No single individual can have the time or the intelligence to do the job.

Do you agree with Johnson and Blair?

There seem to be a few points here that need further clarification:

• Why is it that being an independent rational agent has to mean that one has to confirm each belief candidate independently? As a scientist, for example, I don't have to replicate every single experiment. I can rely on experimental results reported in professional journal. Of course, I still read the articles carefully to make sure that the methodology is not flawed and that the reported conclusions are well-supported. Am I not being a rational agent in doing so? In other words, why does the choice have to be between believing only your own eyes, so to speak, and relying on others? Isn't that a false choice?
• I can accept a method for testing beliefs even though, upon some reflection, I realize that it is ultimately unjustified. For example, we all use inductive reasoning. However, as David Hume argues, we have to realize that induction is ultimately unjustified in some fundamental sense. In using induction, then, or any other method, am I not being a rational agent? In other words, why does the choice have to be between accepting a belief-testing method without scrutiny and endlessly scrutinizing a belief-testing method? Isn't that a false choice?
• Why do I have to "verify everything" in order to be an independent rational agent? Perhaps I don't have the physical and mental power to discover new truths in every domain of knowledge. However, once a discovery is made in a certain domain of knowledge, I can engage in some critical reflection about the nature of that discovery. For example, if I read a report about the Alvarez theory of dinosaur extinction, I may not be able to understand the technical details (e.g., concerning iridium levels and quartz particles). But I can still exercise my critical faculties in evaluating the report, e.g., I can use Ronald Giere's program for evaluating theoretical hypotheses.

From Understanding Scientific Reasoning (2002).

The basic point, then, is that critical thinking is not an all-or-nothing thing. Rather, it is a matter of degree. What do you think?