If you want to get an idea of the intellectual rigor of the different professions, the chart below may be a good place to start. This comes from Mark Perry, an economics professor at the University of Michigan. It represents Graduate Record Exam (GRE) scores broken down by the field of intended field of graduate study. It is based on 2002 data, so is a little dated.
Several things have been pointed out elsewhere: namely, that quantitative fields may be overstated (meaning that fields like philosophy may be understated), and fields in which students tend to take tests other than the GRE may be understated (The better business students, for example, ten to take the GMAT in order).
Also, since this data is based on the intended area of graduate study, it doesn't necessarily comport with the field of undergraduate study, but it should be pretty close.
In any case, not the bottom two categories: education and public administration. That explains a few things. It also may add credence to my observation that an education major may be the closest thing in the academic world (other than maybe public administration) to a lobotomy--or the best evidence that one has already been performed.