The real fire that burns you is the fire that is produced by God as the natural regulatory forces of nature. While the fire that is hallucination is fire that is conjured through the ideation of finite spirits such as other individuals. Real fire, since it is a subjective creation of God, has the ability to burn us, while illusionary fires do not have that inherent ability. Therefore all objectives that are not perceived by other human beings are perceived by God and have an existence within the world.
Berkeley's fundamental argument about reality and matter is that they are all sensory perceptions. However, since God creates ultimate harmony within the world and moreover provides a system in which we live in, his rules applies to all objects that we possess and use. Therefore, although arsenic in itself is nothing more than an idea, it is an idea that is regulated by the ideation of God, and therefore it is part of a defined regularity that have specific pre-defined properties. Eating arsenic, since it has dangerous properties that God instilled in it through His ideation, it is still toxic and deadly to human beings. The role of science has a unique position within Berkeley's philosophical system. Since he argues that the true cause of any phenomenon is a spirit and that the spirit in relations to most objects and phenomenon is God, it would appear as if this is against the grain of established science. Berkeley argues that science is about the discovery of regularities in our ideas. The point of scientific inquiry is to reveal the regularities within the system that God has created for us. Therefore, we know that fire heats and the hearts pump blood to the rest of our body; these discoveries are all part of the universal collection of regularities that occur and are dictated through God's ideation process. Science is about uncovering all of the specific regularities that are associated with ideas.
Thus Berkeley sees scientific explanations not as causes but as signs that point to established universal knowledge. Since the materialist system is created by God, the results of scientific inquiry are useful. Scientific explanations are reductions in regularity rather than actual causal explanations for phenomenon.
Berkeley articulates that God, Himself is a spirit that has infinite rather than finite like human beings. His explanation for why God exists involves a careful explanation on the existence of objects. He operates upon the premise that matter does not exist, and therefore only three aspects of the universe actually exist, other ideas, your own spirit and the spirit of others. Ideas in and of themselves are passive, they have no power or agency in and of themselves, and therefore they possess no causal power. At the same time he argues that while individuals themselves can formulate ideas, sensory ideas in and of themselves are involuntary. They present themselves to the senses not through active thought but unconsciously and without provocation. Therefore, he argues, our sensory ideas must be caused by some other spirit. Since our senses are extremely complicated and very systematic, Berkeley argues that it cannot be conceived of by a finite spirit. Therefore the only thing that could regulate all the things that we sense through our sensory perception must be conceived by an infinite spirit. This infinite spirit in question would be the wise and benevolent God who conceives of all things that we perceive on a natural level. This theory conveniently allows Berkeley to go around the problem of actual materialism by articulating that all things that are physical phenomenon are actually the ideation of God.
One of the hidden assumptions that Berkeley makes within this argument is that anything is caused within the real world must be done so by a willing mind, and therefore an active consciousness must be dictating perception. If we cannot consciously conceive of something then it must be the operative domain of God. However, thought itself and subjective perception in general can occur on the subconscious level, just as we breathe oxygen subconsciously. This is one of the logical omissions within Berkeley's argument. However, on the whole if one were to accept his original premise that matter does not exist. His argument on God is convincing in that it makes sense for the ideation of all things natural and systematic to be in the domain of an infinite spiritual force.