The Thoughts Of All Men

While I was reading Poem 17 of Walt Whitman's Leaves Of Grass, it started out as "These are the thoughts of all men in all ages and lands-". I thought of it very curious that a poem starts like that, Then i kept reading and at the end there was another line that caught my attention again, "This is the grass that grows wherever the land is, and the water is;/ this is the common air that bathes the globe." The poem talks about things like if its not as much yours as mine, then it is not the same. It was cool that the guy talked about his thoughts being the ones of all the world, and then comparing it to the grass, because the grass grows wherever there is land, and he says that these thoughts grow wherever there is people.

"These are the thoughts of all men in all ages and lands—they are not original with me;
If they are not yours as much as mine, they are nothing, or next to nothing;
If they are not the riddle, and the untying of the riddle, they are nothing;
If they are not just as close as they are distant, they are nothing.
This is the grass that grows wherever the land is, and the water is;
This is the common air that bathes the globe." Most of the poems talk about grass, this one is no exception. I wonder what does Whitman mean when he talks about the thoughts of all men. He says "they are not original with me;", which I am guessing makes reference to the fact that he is not the creator of the thoughts, he is just another individual man with the thought, another leaf of grass.