Science works by uncovering certain conditional statements such as “If a occurs under certain ideal conditions, then b will occur” or “if a is present under certain ideal conditions, then b will be present.” For instance, every time water accumulates energy in the form of heat, once the water reaches a certain temperature (approximately 100 degrees Celsius) it will become gaseous. Thus, we can form the conditional statement that “if water reaches the temperature of 100 degrees Celsius, then it will become steam.”Now this is generally how laws of nature come to be formulated. We run an experiment a few times under ideal conditions, and when a certain state of affairs reliably obtains, then we assume that this happens across the board and formulate a law of nature. This means that laws of nature are really just descriptions of the way nature tends to behave.
But what, then, is the explanatory use of laws of nature? For example, if one asks, per our example above, why when water reaches a certain temperature it subsequently tends to become gaseous, it won’t do any good to revert back to the law that states this very thing. For that would simply be to say that a tends to be followed by b because a law of nature says that b’s are reliably produced by a’s. Such an answer would thus amount to a tautology, and be explanatory vacuous.Now, one could instead say that the boiling point of water is not explained by the law of nature, but rather that the boiling point of water is explained by molecular behavior, kinetic energy, and molecular forces. The problem, though, is that here a law of nature is simply being explained by more laws of nature—for molecular behavior, kinetic energy, and molecular forces are themselves simply physical descriptions. That is, this explanation is tantamount to saying that the reason a’s are followed by b’s is because c’s are followed by d’s, and e’s are followed by f’s etc. But, this only pushes the problem back a bit. For the original question was not how water becomes a gas, but, rather, why it does. And explaining laws of nature by other laws of nature only answers a “how” question, and not a “why” one. (Not to mention that if this method of explanation were valid, then eventually we would arrive at a law of nature which simply has no explanation whatsoever, which is absurd.)
This is to say that the ultimate question here pertains to asking why things tend to behave the way they do at all. And again, it will not do to answer this by reverting to any law of nature, because a law of nature is purely descriptive. But then what tool does science have for answering this “why” question, since all science can really do is formulate descriptive laws of nature useful for prediction and manipulation? The answer is that science simply does not have this tool, philosophy does. And this is why naturalist Bertrand Russell said the following:[Physics] lays down certain fundamental equations which enable it to deal with the logical structure of events, while leaving it completely unknown what is the intrinsic character of the events that have the structure[…] All that physics gives us is certain equations giving abstract properties of their changes. But as to what it is that changes, and what it changes from and to—as to this, physics is silent.
The point here is that science can tell us about certain regularities in nature. But, it cannot tell us why these regularities happen without fail, or why the correlation between these regularities is necessary, or exactly why it is of the nature of substances to have certain dispositions to behave the way they do, or even why there is regularity at all.Thus stated, we see that science does in fact have explanatory limits, and therefore we should not expect that science exhausts our knowledge of reality. Science is not the be-all end-all description of reality. It tells us nothing about the intrinsic nature of the things that it describes. It only tells us of the quantitative, and not the ontological.