It is the philosophy of sages who bond with nature. The act of letting go and relaxing the will enough to unite with the environment. It is a selflessness that only the strong in heart can achieve. All the pure sages Hagoromo Asura Hashirama Jiraiya Minato Naruto they all share a personal characteristic of being the one is willing to sacrifice and help others. This is a quality of the chakra they produce. It is the will of the chakra to connect with others. So what im saying is that the will of fire is senjutsu or the way of senjutsu long before sage mode ever happens.