The sentence 'Seeing is understanding' is correct and usable in written English. You can use it to explain that gaining firsthand experience is key to understanding something. For example: "Seeing firsthand the struggles of those living in poverty was essential to understanding the issue.".
Rudolph Arnheim, a great theoretician of culture, had already predicted in the 1930s, in his book Film as Art, that human beings would come to confuse the world perceived by their senses and the world interpreted by thought, and would believe that seeing is understanding.
The "seeing is understanding" method of analyzing data is frequently employed by professional traders or investors with access to expensive software.
Moreover, the audio-visual information that teleconsultation can provide should, according to one respondent, be viewed with scepticism, as "the directness and ease with which we see could lead to the misconception that seeing [alone] is understanding".
"The best thing about having now seen the movie is understanding the plot – I had no idea what I was doing during filming," he told talk show host Graham Norton.
Charles seems to be a husband who dislikes his wife and is constantly told by her what to do, so he is surprised to see that Marge is understanding and kind.
From what I understand, his boyfriend is understanding or might see it as a phase in their relationship.
Sometimes seeing is believing -- or understanding.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union