Might be made up quote:
Experiments are the only means of knowledge at our disposal. The rest is poetry, imagination. As quoted in Advances in Biochemical Psychopharmacology, Vol. 25 (1980), p. 3
That’s as far back as I can find the quote so far.
Got a little closer:
Zeitschrift für philosophische Forschung – Volume 13 – Page 119
Die Bedeutung der Phantasie hat Max Planck, ebenso wie Einstein, immer wieder aufs stärkste unterstrichen.
The importance of imagination, Max Planck, as Einstein repeatedly emphasized most strongly. He said in his … Without this ability of our imagination we would have neither poetry nor art
So – so far I can see Einstein quoting Planck… hm.
Planck wrote about “Imagination and Faith”. I’ve seen “Phantasie und Glauben” as a common theme in 19th and even 20th Century German Scientific writers, including a geophysicist named Schmidt, who would have been older when Planck was younger and likely influential. I saw the phrase enough times together that it seems to be somewhat of a “Scientist Theme” of the time, at least in part.
By Faith, it means a trust, a hope, a leap into the unknown. It doesn’t include or exclude a religious notion of faith either; it was speaking primarily in the context of the Scientist
I don’t think Planck was dismissive of imagination and poetry at all. Einstein, in fact, seemed to portray Planck as a man of great imagination. But rather, I think Planck’s thing was: Know the difference, please.
You can trust Experiments.
Anything else? You can’t trust completely. Test, poke, prod, etc.