Board Thread:Theories and Speculation/@comment-26388300-20150523085628/@comment-26388300-20150619174204

My understanding of Hashirama's senjutsu was this:

Senjutsu is the chakra of nature... nature, not animals. That said, animals, generally, are more in tune with nature than humanity. As a result, it comes as no surprise that some of he more advanced animal kingdoms in the Naruto canon have harnessed senjutsu and are capable of teaching it to shinobi.

That said, not all senjutsu, I believe, must be of the variant that animals have harnassed. Hashirama was a masterclass shinobi, a lover of peace, and a wise man; it is rather easy to reckon that he removed the middle-man, receiving no training from an animal summon, but was attuned to nature himself so well that he constructed his own variant of senjutsu... Senju Senjutsu, if you will. Therefore, it lacks any distinction of a particular summon.