Lucretius’s first move in a defense of Naturalism is to identify the real with the object of touch, and so with what either gives resistance or not. What resists is extended and divisible, and so is analyzed into what one finds after all possible divisions have been made (or the a-tomic) what does not resist is void.
The intuitions Lucretius is working from here are so fundamental that we haven’t yet come to terms with what it means to overturn them. All sensation seems to be just various ways of being struck by things, and all bodies are fundamentally inertial and are busy falling toward one another. This is what Newton saw in the apple.
For all that, our attempts to limit nature to the (even in-principle*) tangible have failed. What resists has mass, and much of the cosmos is massless without being void. I don’t know that we’ve come to terms with the scandal that this presents to our attempts to limit the real to the tangible (or to anything that is even in-principle sensible) but this is where we find ourselves. We can lean heavily on the mathematical description of the massless to diffuse the scandal, but at some point we’re going to have to accept that sensation sets no limit on what can be known even about natural things. This causes a re-alighnment of ancient physics as much as contemporary Naturalism.
*By “in principle” I mean that one can’t just maximize sense power to the point of detecting them.