How to cite this quote "The whole is other than the sum of its parts" in an article
It sounds like you are effectively asking if it is OK to cite a secondary source, if the primary source is not able to be located or retrieved. This varies by publication style guides; for example, in IEEE secondary sources are (at least in theory) prohibited, though as far as I'm aware there are no Grand Inquisitors enforcing this rule, so if you do it I don't know that anyone would think that an adequate reason to suggest rejection of your paper...but I haven't tried it.

As you mention Psychology, I'll note that in APA format, the answer is: yes, it absolutely is OK to use secondary sources, so long as you keep them to a minimum. However, your use of the brackets makes me think you might be using an ACM/IEEE style, in which case you are not supposed to use secondary sources; either you find a primary source, or you simply do not cite it at all. 

If ACM/IEEE is your style, I would tend to suggest you just say something along the lines of, "in the words of the Gestalt psychologist Kurt Koffka, the whole...". And then you just don't cite it, because it's clear you are using someone else's words, you don't have a primary source, and it would seem weird to me for a reviewer to object to there not being a citation to that unless your work absolutely depends on Kurt Koffka having said those words (which would make no sense in a field that uses ACM/IEEE).