There is a debate whether or not demons exist or existed. Not to get into the huge topic too much (since I don't know all the sources and one could write a whole book on the subject) but I came across a Midrash Tanchumah (MT) which I have not heard before, so here it is:
The MT in parashas Naso, sif 23 states:
"...on the day the Mishkon was erected, the demons were eliminated from the world. Before it was erected, the demons were commonly found in the world."
The Midrash then proceeds to cite proof-texts as well as some descriptions of various demons and then concludes "the day the Miskon was erected they were eliminated."
Now from what I remember, the RamBam and Meiri (along with some others) say that demons do not exist. The question some raise, is that are they saying the demons never existed at all? Or that they simply do not exist today? From this Midrash, we can surmise (possibly) that they did indeed exist at one point, but were eliminated form the world after the Mishkon was finished.
Therefore, when a reshon states that demons do not exist, it could be that they are speaking only of the present and future.
