The mention of gravity leads into a discussion of how Philonous' idealist thesis coheres with science — in particular, with issues of scientific truth and progress. Hylas claims that Philonous' idealism cannot possibly stand up in the face of the enormous and obvious progress that science has already begun to make. Since this progress has been made precisely by postulating the existence of unperceivable material entities that are described in terms of a purely mechanical, observationally testable physical theory, how can Philonous doubt that materialism is true? Philonous argues, in response, that none of these recent scientific explanations actually depend on the existence of matter. What these scientific discoveries show is a constant conjunction between various of our ideas, nothing deeper. For instance (to use an example Philonous does not use himself), when scientists discovered the connection between heat and molecular motion, they were not discovering that the motion of material molecules in a mind-independent material object causes the sensation of heat in perceivers. Instead, they were merely discovering that the sensation "seeing tiny particles move" was constantly accompanied by the sensation "feeling heat". In other words, what science does is to discover patterns in our ideas. This is very useful, but it should not be overestimated: science is not getting at any deeper level of reality. Sensations are as deep as reality goes.
Hylas then asks an obvious follow up question. If science is not getting at a deeper level of reality, why do scientists use microscopes to try to discover what things are really like? What would be the point, if there is nothing deeper to reality than our sensations? Philonous explains that when we use a microscope to investigate some object, we are not finding out what that object is really like, rather, we looking at a totally different thing from the object we placed under the lens. For instance, say you put a piece of cork under a microscope lens. When you look through the microscope you see a complex arrangement of cells, totally unlike what you saw with your naked eye. Materialists want to say that what you are seeing now is the microstructure of the cork. Philonous, on the other hand, wants to say that what you are now seeing is a completely different object, not the cork at all. (Because you are having a very different sensation.) However, this object has clear relations to the cork, and the purpose of looking in the microscope is to figure out this relationship. In other words, the purpose of using a microscope is the same as the purpose of all science: to discover the pattern between our different ideas. The more we know about how our ideas are connected together, the more we know about the natures of things.
The same is true, Philonous continues, of the ideas that we receive through different sense modalities (i.e. touch, sight, sound, smell, and taste). We do not see the same object that we feel, we do not feel the same object that we hear, and so on. Each of these objects is distinct, but they are closely related because God always presents them to us in a certain fixed pattern. The reason we speak as if all these sensations were of the same object, is merely for the sake of convenience. It would be needlessly complex if we had different names for each of these objects, and needed to track each one separately. So instead we speak as if the cherry we taste is the same as the cherry we see, and the cherry we touch, and the cherry we smell. In reality, though, none of these is the same object. The case is exactly the same for our sensations at different times: if I see my house today, and then again tomorrow, I am not really seeing the same object at all. However, for convenience's sake, we act as if this were the same object at all times of perception.
In addition, no two perceivers can ever see the same thing, since the idea that is in my mind cannot be in your mind and vice versa. This does not mean, though, that my experience of the world is unlike anyone else's. The ideas that I perceive are indistinguishable from the ones you perceive; they simply are not the same ideas in the technical sense of being one and the same thing. Philonous points out that though this feature might seem unattractive, it is not unique to his theory: his materialist opponents also believe that what we immediately perceive are our own ideas (remember that Descartes and Locke both held a mediated view of perception), and so they run into the exact same problem.
So, Hylas asks after all of this has been laid out, does this mean that God is a deceiver? This picture of the world is certainly different from what we come to believe just by looking around us. Absolutely not, Philonous responds. God would only be a deceiver if he either revealed something false to us through supernatural revelation, or else if he made the deceptive opinion so perfectly evident, that we could not help but believe in it. But God did not do either of these things. Nothing about the way the world is presented to us indicates that reality is different from what I have just described. In fact, it is only the philosophers who got things wrong. Everyone else comes very close to having the right ideas: namely, that what we perceive is what exists.
The new mechanistic science of the 17th century had been having stunning success in the years preceding the publication of the Dialogues. Newton had made and presented his most important discoveries in physics, chemists were unraveling the inner workings of nature, and engineers were inventing amazing machines. And all of this progress was coming on the heels of the already astonishing accomplishments of the previous century, in particular, the work of Galileo. Given that all of this progress was being made on the basis of assuming a materialist hypothesis, Hylas is right to point out that it poses Berkeley's greatest challenge. Yet Philonous remains unfazed by this challenge: not only does the success of the new science fail to refute his idealism, he claims, but the metaphysics of his idealism actually meshes better with the new science than the metaphysics of materialism does. It is worthwhile to look at these two claims in greater detail, fleshing out the theory of science Berkeley presents in the Dialogues with the related thoughts he presents in the Principles and De Motu, his work on force.
The claim that the new science does not require a commitment to materialism can actually be broken down into two further claims: (1) The observations, predictions, and descriptions that empirical scientists have been making are perfectly compatible with the idealist view and, (2) The claim of these scientists to be discovering the microstructures that explain macroscopic phenomena are also consistent with idealism. We have already seen how Berkeley backs up these two claims: he asserts that what the scientists are discovering in both cases, are relations, or patterns, among ideas. The scientist observes connections among actual ideas, and makes predictions about future ideas. Since the connection among ideas are lawlike (that is, they constantly hold true), they allow us to predict with regularity what ideas we would have if we were presented with certain other ideas (for instance, that if we were presented with the idea "quickly moving particles" we would also be presented with the idea "heat"). When the scientist looks in a microscope he is observing some very particular sorts of ideas. God has set things up so that there are certain of our ideas that make up micromechanisms, in terms of which macrophenomena can be mechanically explained. Observing these micromechanisms provides us with a whole new law-like system of connections and regularities that we can use in order to predict and control the natural world.
Berkeley goes on to explain why this idealist interpretation of the new science is actually superior to the materialist interpretation. First of all, as he has already shown, materialism leads to skepticism. By claiming that there is some real essence in objects (i.e. some internal qualities or constitution) that is hidden from our view, the materialist interpretation implies that there is a limit to our knowledge. According to this view, we cannot comprehend everything there is to comprehend about the natural world. Locke himself showed where this limit comes in: we cannot understand how the internal constitution of objects gives rise to the secondary sensible qualities we observe, such as color, taste, smell, and sound.
The materialist interpretation is needlessly complex. By including matter in the picture, it adds an element that can do no explanatory work for us. Matter is by its nature something that transcends our experience (since all we experience are sensible qualities, and these cannot belong to matter), but what we have to explain in science is the behavior of the objects of our experience. Matter, then, cannot help us to understand the world any better.
Hylas is satisfied with this two-part response to the challenge from science, but should he be? Many problems still remain with Berkeley's account. First of all, Berkeley's interpretation seems to discount the importance of scientific truth. We do not just want our theories to make accurate predictions; we want our theories to identify the real causes of the phenomena we observe. We want our theories, in other words, to describe the way the world really works. But on Berkeley's view, our scientific theories cannot do that. The way the world really works, according to his idealism, is that God controls all of our ideas according to certain rules. We might be able to figure out these rules well enough to predict which ideas will follow which others, but we will never understand the reasons for these rules, and we will never really understand how and why our ideas are produced in the order and manner in which they are. Science will always only get us as far as our own ideas, and never to the controlling God behind them, and so it will never tell us how the world really is.
We also believe in scientific truth when it comes to the microscopic level. We think that when we discover how the microstructures of objects are organized, we are getting at the inner workings of things; we do not think that we are just adding more ideas to the mix. We think that the microstructures are actually a part of the macroscopic object, not just further ideas that are somehow correlated with that object in our experience.
Given that Berkeley thinks that microstructures are simply further ideas, we might even ask why he thinks they exist. Why would God even make these ideas of molecules and atoms? These microscopic ideas, which must then be correlated with the macroscopic ideas, just needlessly complicate the world. Why would God not make the world simpler, so that we could just look and see the laws of nature are with our bare eyes? Berkeley tells us that these microscopic things exist because God thought it would be best for them to eixst, but this is no explanation, just an assertion.
Berkeley's reply to these worries concerning scientific truth is to point out that the materialists are in no better position than he to give us scientific truth. Scientific knowledge, according to Berkeley, Locke, Descartes, and all the other philosophers of that time, meant knowledge of necessary connections: in other words, in order to understand how A caused B, for instance, you would have to understand how A could not failed to have caused B. But Locke himself claims that such knowledge is not available to us concerning the natural world, because we cannot understand how the microstructures of objects give rise to their macroscopic properties (in part because we cannot observe these microstructures, and in part because there are no such necessary connections to even be observed in the case of secondary qualities). Since the materialists cannot discover necessary connections, they cannot get to any deep scientific truth. All that any of us, materialist of idealist, can do is to empirically gather information about what observable qualities are constantly conjoined to what other observable qualities.
This reply of Berkeley's is inadequate on several levels. First of all, contrary to what Locke claimed, scientists have been able to discover necessary connections in nature: the identification of heat with molecular motion is one example of such a discovery. Second of all, whether or not it is true that scientific knowledge in the strictest sense depends on necessary connections (which is itself a dubious claim) it is certainly not true that science needs to uncover necessary connections in order to tell us what the world is like. We think, for instance, that Darwin's theory of natural selection tells us what the world is really like, even though this theory involves no necessary connections, but only probabilistic connections. Similarly, though we have yet to discover the necessary connections lying behind genetics, we think that that field has already gone a long way toward telling us what the world is like. Another worry we might raise for Berkeley is this: most of us tend believe that it is possible that science could require us to postulate the existence of in principle unobservable entities: entities that we know exist, because of their explanatory power, but that we will never be able to observe, no matter how advanced our microscopes become. (For a while, physicists believed that neutrinos were such an entity, though later they did find a way to observe them.) On Berkeley' system, we could never posit such an entity, because an object that cannot be perceived is an object that does not exist.
In Berkeley's De Motu, he discusses Newton's laws of gravitation. Like Newton himself, Berkeley did not think that Newton had discovered a new force in the world called "gravity". Instead, he thought that he had discovered certain mathematically expressible laws that we could apply to the objects around us in order to predict their behavior. "Gravity" on this view, is just shorthand for some mathematical equation. Berkeley, we can assume, would treat in principle unobservable entities in the same way: not as really existing things, but as useful fictions which help us to make predictions. This is a view that certain scientists and philosophers of science hold today, but most people find it very unattractive, and so might find Berkeley's theory unattractive for requiring it.