I just found an article by Minor White concerning the subject 'equivalence' here:
http://jnevins.com/whitereading.htm
Personally I am not sure if I am feeling the same as Stieglitz did, when looking at his pictures, but I think I can understand the concept.
I don't think that one has to reach out to a metaphysical argument for explaining, why people may feel something similar when looking at a photograph. Maybe it has more to do with shared experiences as human beings, empathy and structural similarity than with 'spirit' or 'essence'.
The problem with the metaphysical stance is that one has to explain why others don't feel the same as the photographer intended to. Saying that one is not emotionally, kinesthetically and intellectually sensitized enough instead of critcizing ones own conception may give the impression of vanity.
However, I think that being aware of the subjective components when taking a photograph can have an impact on ones photographs.
There are some other interesting articles and interviews on that site (Weston, Frank, Winogrand, Strand, Szarkowski, Smith).
If I knew, would I take the pictures I take?
Frankly, I do not know. Maybe you should try it?
😉