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.