last 14 lines in Bishop's poem

The last fourteen lines of Bishop’s poem describe the salty ocean that is a “transmutation of fire.” Bishop equates “knowledge” with this acidic like water. Bishop explains “what we imagine knowledge to be” through opposites. For example, she describes knowledge and the ocean as both “dark” and clear.” I thought these were rather opposite words to be listed in the same line together. I think the author does this because just as the ocean is not stagnant, knowledge is forever “moving” and “utterly free” to change. Our perception of truths is constantly in flux. Also, knowledge can be “bitter” and “burn” just as the salt from the ocean stings the skin. The last line of the poem couples nature and knowledge through “our knowledge is historical, flowing, and flown” because obviously water flows but knowledge flows too through generations and perhaps the “flowing” refers to blood flow and the idea that knowledge grows and moves through human beings. The tense of flow starts as the present tense; “flowing” because living and carrying knowledge is present but knowledge is also “flown” because in a “historical” sense knowledge has been previously attained; therefore, Bishop uses first present tense, then past tense to reveal knowledge as existing through time and society.