"Spirit"
It is really a tough word to nail down. Some folks use it interchangeably with the word soul. I can also be used to state incorporeal mind. For others the word means something like the wind from which an action is originated. Like acting out in the spirit of benevolence. Some Native Americans used it in this way, even though it was simultaneously a reference to the driving force behind nature. "The great spirit". He says "some other spirit".
So, it would seem that he may simply be saying a wind other than that of man's self perceived wind. Man may find it repugnant that his own wind ,ideas formed by reflection on sensed reality, is capable of the complete unfathomable movements seen in nature. This doesn't positively state that there is no will higher than that of the one behind nature.
In some understandings man's naturally stimulated reaction to environment is that of a creature, similar to plants and animals. This would be flesh or beast. Then there is the concept of absolute being beyond the state of reacting within the stimulation of nature. This being is often referred to as the power behind nature's manifesting purpose, simply put "will" or "spirit". The middle ground between these two is considered the place of ensoulment. The soul of something when the will has resulted in a manifested thing, matter acting out the cause of will, or even the encapsulation of spirit in the confines of a measurable experience.
The use of nature in this way has the hint of many a pagan cultures. However, while only expounded on to this extent, it does not necessarily oppose Christianity. Perhaps some secular dogmas would be repulsed by any use like his, but it isn't universal heresy.
What he has done, is declared that everyone can see there is something more causing natural activity than the sensation of it. This invites those that don't want to believe in an entity or grand persona, lording over all existence, to use the term God as something that science might label as the originating cause behind phenomena. Some minds prefer to avoid the idea of incorporeal bodies. So, if you prefer the ideal of unseen powers, like magnetism, working in a unison to bring about natural reaction, you can be free from entanglement of a spiritual mindset. He has not said that the will behind manifestation is a mindless accident. He has just said that all men should be able to recognize a driving force beyond that of man's sense and reflecting capacity.
Some of his deeper considerations are that the world of nature is the sensing of reality interacting with mans self realization. This is also sometimes a distressing thought for theologians. But, again he still has the convenience of saying the intent behind the interaction is due to the planed intent of some being other than man.
It seems like he is saying that nature is only there for the sake of our personal interaction with it. He also heads in the direction that nature is only really there in sensation. Sensed reality is only manifest to the degree required for us to actively participate, and has no real foundation beyond our sensing capacities or the energy that produced it. This encapsulates all of our scientific explorations, into the study of a fine crafted fractal. This is how he can get away with it. He submits that there is undeniably an interaction, which sense and reflection are a vehicle for. This, in some ways, undermines the use of analyzing nature to declare a mindless or accidental causation. This is done by pointing out that you are searching the perceived world for proof of mind, when you should be looking for the mind in the architecture of the laws governing your experience.
His pursuit of this direction has something to do with influencing the reasoning behind societal structure. If you use the observation of nature to determine the appropriate direction for mans efforts, you will only encourage the pursuit of sensations and the advancement of man built on endless fractals. His possible argument might have been that societal structure would be better directed towards integrating with the will that set the stage, rather than the stage itself.
Myself, I think that the danger he faces, in this line of thinking, is the door it opens for gnostic considerations.
Anyway, I probably overlooked something. But, this is the understanding I have gathered by my brief exposure to Berkeley. I admit I have read only a smaller portion of his material, and commenting in length exposes me to inevitable error. What can I say? I like it when it hurts
