Proper science is always descriptive. It seeks to be predictive. It never is prescriptive.
I believe that confusing these categories is what is behind the muddled debate regarding the role of science and faith in Scripture.
Before looking at the three categories, I think it is important to identify the fundamental premise of the discipline of science. This premise, which is necessarily an article of faith, is the belief in the uniformity and consistency of nature. Similar conditions produce similar outcomes. Science must assume this a priori. The entire discipline may be summarized as an empirical search for confirmation of this premise. As David Hume pointed out many years ago, empirical observation can never prove an absolute truth. In order for science to work as science, the scientist must assume that nature behaves consistently. The thoughtful philosopher of science acknowledges this. He is comfortable working with the tools of science while keeping in mind the scope of his exploration. Less thoughtful scientists deceive themselves when they attempt to bootstrap conclusions from observation as justification for holding the premise. This self deception rubs off on the world as well.
The descriptive role of science
We often hear of the fundamental laws of nature. Indeed, a scientist’s professional aspiration is to discover some fundamental law of nature. A thoughtful scientist acknowledges that a fundamental law of nature is a systematic description of observations. In other words, the scientist acknowledges that a law of nature is a man derived discovery, not a decree. However, many confuse this distinction.
One example of this was a conversation I had with my soil physics professor when I was studying advanced soil physics at my university. We were all familiar with what is known as Charles’s Law. This is a natural law that (among other things) describes the relationship between temperature and pressure of a gas. Generally speaking, if volume remains the same, when the temperature decreases the pressure of a gas decreases. This observation has been confirmed over and over again to the point that it is accepted as truth. In reality, it is a professional "rule of thumb," because real gases depart subtly from the ideal model.
The professor was discussing an interesting way of measuring vapor pressure within a plant by use of a thermocouple psychrometer. The device used a microscopic thermocouple placed inside a root or a leaf of a plant. Voltage was applied to the thermocouple, and through what is known as the Peltier effect, the temperature of the thermocouple element would decrease. At some point water droplets could be detected on the element. By knowing the voltage applied to the thermocouple, you could determine the temperature at which the droplets were formed. Through a series of simple calculations, applying Charles’s Law, you could determine what the vapor pressure of water was inside the leaf or the root prior to the experiment.
As the professor was describing this experiment, he asked us why the water droplets formed. I answered that it was because the temperature drop had reduced the heat of the system and therefore the number of collisions of molecules was insufficient to maintain the water in vapor form, so they condensed. The professor shouted, "No! It is because of Charles’s Law!"
My reflexive response: "I am sure water molecules behaved this way long before Charles had been born."
My relationship with the professor deteriorated after that point. He doubted that I would be a good scientist because I refuse to hold a natural law as foundational.
The predictive role of science
Having stated that the laws of nature are summary descriptions of observations, we can briefly consider the most powerful and useful aspect of it: its predictive role.
When a scientist gathers sufficient information to propose that he has discovered a law of nature, the test then rests on whether it can predict a certain outcome. This is seen in every area of science. For example, you come up with an idea that it takes a certain amount of heat to bring water to a boil. You measure the amount of heat applied to a small volume of water and then come up with the idea that a larger amount of water requires proportionately more heat to boil. If you can measure these things carefully and discover the relationship holds true, you become more confident that this will hold true in other cases. Other observers try the experiment and confirm the result. It becomes accepted as a proper model of the behavior of heated water.
Similarly, if you develop a system that accounts for the relationships of planets and stars, you can use that to predict a solar eclipse. Even though the system does not explain why planets and stars do what they do, it accounts for their behavior. It is a systematic summary that consistently predicts.
No question this ability to predict future behavior is a very powerful and useful. All of our technology, from something as simple as hammering a nail to very high-frequency transistor switches controlling cars, nuclear reactors, or deadly missiles, depends upon predictability. This predictive ability of science gives us its dazzling stature.
But while we are dazzled by the predictive ability of science and what technology has brought about, we must not forget the underlying premise: the uniformity of nature. All technical advantages and advances are based upon the fundamental assumption that matter and forces behave consistently. The fact that science has been so successful demonstrates the reasonableness of this assumption. But we must always remember that the reasonableness of the assumption, or the success of science, does not prove the assumption.
Science is not prescriptive
Here is where we find confusion. Because science, at its foundation, rests upon empirical observations, it can never explain the ultimate why. There is always the possibility that somewhere out there lurks an exception. A good scientist acknowledges this and seeks to test probability of this happening. But even this is limited by the number of observations one can make.
Some will forthrightly state that the teleological has no place in the inductive discipline of science. This is a fair statement, if consistently believed. If the scientist at the beginning consciously chooses to assume that there is no purpose behind a particular set of observations, he has foreclosed his option of asserting that there is no purpose elsewhere, because he has refused to look for purpose at the outset. As long as the scientist understands this fact, he is unlikely to be confused by his own conclusions.
But people, including scientists, can be blinded by success and forget their original commitments and assumptions. Because a particular model is successful in explaining and predicting future behavior of matter and energy, it is easy to fall into the notion that such an outcome must occur. But this is actually begging the question. Asserting dogmatically that an outcome must occur because it has always occurred in the past is merely incorporating the original premise into the conclusion. In other words, you do not prove your premise by saying things always occur in a certain way because they always occur in this way.
This bootstrapping of the premise into the conclusion is where modern understanding of science goes astray. It is one thing to come up with a model saying that the force of gravity requires planets to orbit around the sun in a certain fashion. It is quite another thing to say that this is why a planet orbits the sun the way it does. Nobody has explained why there is gravity, or even what it is about the mass of an object that gives it a certain amount of gravity. This is only taken as a given. So far, we can only say gravity exists because it exists. True, you can measure the force and you can use the strength of that forced to predict outcomes, but you have not explained the force.
The scientist contemplating gravity, magnetism, electromagnetic forces, or any other property of what is found in creation, is really no different from the primitive man who may have observed the fact that the sun comes up in the east every morning. In both cases, the assumption of uniformity is based upon observations taken at face value. In both cases, the men accept the observed facts without explaining them further. And, in both cases, the answer to the question of "why must this be?" can be legitimately answered by "because God made it this way."
The scientist, atheist or not, can have no rejoinder to this answer. He can refuse to account for God as creator and develop a system accordingly. But, because the system is closed and limited only to a fixed set of observations, he cannot rebut it.
I grow weary of good-faith attempts by Christians who try to refute science by using science. You can never use science to explain the ultimate why. Accordingly you can never use science to offer an alternate explanation of the ultimate why. The question is simply outside the scope of the system.