Meaning of this phrase which has old text structure

I'm translating an old English book that's about philosophy and I came across this paragraph which I have trouble understanding.

If we understand that reason derives abstract units out of concrete multiplicities, that it constructs truth out of phenomena, substance out of attributes, that it perceives all things as parts of a whole, as individuals of some genus, as qualities of some object, then the question regarding a "thing itself," a something which in reality is back of all things, must deeds become irrelevant.

since this book is about philosophy, it has some inherent complexity and it's also old so I encounter some phrases with structures that I'm not familiar with (as a non-native English speaker); like the one here. my major problem is with the phrase "must deeds". I know all of the meanings of these two words, but I can't make sense of them here. Does it mean something like "deeds that are necessary"? or does it mean something like this "all the deeds should become irrelevant"? Unfortunately, I just have a few pages of a PDF file that doesn't entail any source or indication about the name of the book or author. Any help would be appreciated.

A search for the first sentence produced this, The Nature of Human Brain Work. Joseph Dietzgen 1869

https://www.marxists.org/archive/dietzgen/1869/brain-work/ch05.htm

If we understand that reason derives abstract units out of concrete multiplicities, that it constructs truth out of phenomena, substance out of attributes, that it perceives all things as parts of a whole, as individuals of some genus, as qualities of some object, then the question regarding a “thing itself,” a something which in reality is back of all things, must needs [sic] become irrelevant.