“can certainly” vs “certainly can”

Question

This function, at least, philosophy can perform. Most philosophers, rightly or wrongly, believe that philosophy can do much more than this—that it can give us knowledge, not otherwise attainable, concerning the universe as a whole, and concerning the nature of ultimate reality. Whether this be the case or not, the more modest function we have spoken of can certainly be performed by philosophy, and certainly suffices, for those who have once begun to doubt the adequacy of common sense, to justify the arduous and difficult labours that philosophical problems involve.

Problems of philosophy, Bertrand Russell, Chapter II

What does the difference between "can certainly" and "certainly can"?

Answer 1

There are two auxiliaries i.e. can and be in the sentence presented.

You can put the adverb certainly either in front of or after the first auxiliary. So both ...certainly can be .... and .....can certainly be ... are correct.

Answer 2

According to the Cambridge Dictionary, adverbs of certainty or obligation usually go in the mid-position. Where there is more than one verb, mid position means after the first auxiliary verb or after a modal verb.

In this sentence, can is a modal verb (there is no he cans), so after can is the best position- can certainly be.

Having said that, adverb position is very flexible, and certainly can be is not wrong.

The two versions would have exactly the same meaning, but the stress would fall on a different word, giving a slightly different emphasis, The stress would fall on the second word of certainly and can, so "certainly can" emphasises the fact that it's possible, and "can certainly" emphasises the certainty.