For future reference, when we (primarily Don 
) have time, i would like to mark this Latin text as one which has always seemed to me to be ripe for misunderstanding and yet highly useful if we could get a grip on its full meaning.
I have sometimes thought that this might be as straightforward as referring to, for example, the type of person who prefers to speculate with his mind about whether a building is square or round, rather than simply walking over to that building to see for himself up close. Munro, as usual, is relentlessly literal, and Bailey largely follows Munro. I think that the 1743 edition probably is expanding the text in a correct direction, but it may convey a generally right result with a less-than-optimum illustration.
At any rate, this "idiom" comes in the middle of a text that is so important that it seems a shame not to get every last bit of meaning out of the illustration that we possibly can.
So the question is, presuming the Latin is unadulterated, "What does it mean to place one's head where one's feet should be?"
Here's the Latin, from Munro:
1743: Lastly, if anyone thinks that he knows nothing, he cannot be sure that he knows this, when he confesses that he knows nothing at all. I shall avoid disputing with such a trifler, who perverts all things, and like a tumbler with his head prone to the earth, can go no otherwise than backwards. And yet allow that he knows this, I would ask (since he had nothing before, to lead him into such a knowledge) from whence he had the notion what it was to know, or not to know; what was it that gave him an idea of Truth or Falsehood, and what taught him to distinguish between doubt and certainty? You will find that knowledge of truth is originally derived from the senses, nor can the senses be contradicted, for whatever is able by the evidence of an opposite truth to convince the senses of falsehood, must be something of greater certainty than they. But what can deserve greater credit than the senses require from us? Will reason, derived from erring sense, claim the privilege to contradict it? Reason – that depends wholly upon the senses,which unless you allow to be true, all reason must be false.
Munro: Again if a man believe that nothing is known, he knows not whether this even can be known, since he admits he knows nothing. I will therefore decline to argue the case against him who places himself with head where his feet should be. And yet granting that he knows this, I would still put this question, since he has never yet seen any truth in things, whence he knows what knowing and not knowing severally are, and what it is that has produced the knowledge of the true and the false and what has proved the doubtful to differ from the certain. You will find that from the senses first has proceeded the knowledge of the true and the false and that the senses cannot be refuted. For that which is of itself to be able to refute things false by true things must from the nature of the case be proved to have the higher certainty. Well then, what must fairly be accounted of higher certainty than sense? Shall reason founded on false sense be able to contradict them, wholly founded as it is on the senses? And if they are not true, then all reason as well is rendered false.
Bailey: Again, if any one thinks that nothing is known, he knows not whether that can be known either, since he admits that he knows nothing. Against him then I will refrain from joining issue, who plants himself with his head in the place of his feet. And yet were I to grant that he knows this too, yet I would ask this one question; since he has never before seen any truth in things, whence does he know what is knowing, and not knowing each in turn, what thing has begotten the concept of the true and the false, what thing has proved that the doubtful differs from the certain? You will find that the concept of the true is begotten first from the senses, and that the senses cannot be gainsaid. For something must be found with a greater surety, which can of its own authority refute the false by the true. Next then, what must be held to be of greater surety than sense? Will reason, sprung from false sensation, avail to speak against the senses, when it is wholly sprung from the senses? For unless they are true, all reason too becomes false.
