We know that just because event B immediately follows event A does not mean that event A "causes" event B, as we use the word. The occurrence of these two events close together in time (or even simultaneously) and perhaps space may just be "coincidence," two incidents, or events, co-occurring at about the same time. Even if this coinciding seemed to occur fairly frequently, one event would not necessarily be considered to be caused by the other. For instance, they could both be caused by something else. So "cause" refers to something more than just co-occurrence.

So we generally say that event A causes event B if that sequence of events "has to" occur according to the "rules of the universe" ("natural laws"). But what are these rules? They are of course what science seeks. If we know the rules of the universe, then we can predict, at least somewhat, what is going to happen, and we can make things happen, at least to a certain extent, by setting up situations such that the rules of the universe make it predictable that what we are trying to make happen will indeed happen. So these rules of the universe are simply descriptions of what always tends to happen given certain kinds of situations.

(Please note that we are not using "rule" to refer to "that which should be done," an ethical proposition.)

But they are very basic descriptions. They are supposed to always be followed. If there are situations in which a presumed rule does not seem to be being followed, then the assumption is usually made that that rule is only a "special case," applicable to certain kinds of situations, of a more basic rule that indeed always is followed, in all situations. Or the assumption is made that there were also other causative processes occurring that we did not or could not take into consideration.

Scientists are seeking a "theory of everything," which would be a set of rules that would be found always to be followed, in every situation, and such that everything that indeed does happen is simply an example of the operation of those rules. At the time of this writing, relativity theory and quantum mechanics, both highly reliable and valid in certain, different, kinds of situations, are found not to be compatible with each other, in that in situations where one set of rules is followed the other set of rules is not. The "theory of everything" would explain this difference as being the result of a more basic set of rules (superstring theory or M-theory being considered examples) that were indeed always being followed, but differently in different kinds of situations and thus leading to different results, consistent in this example either with relativity theory or with quantum mechanics, depending on the situation. Relativity theory and quantum mechanics would just be special cases of the theory of everything, just as Newton's laws have been found to be a special case of relativity, valid enough in certain situations in which the unusual effects of relativity are so small they are not easily measurable, resulting in Newton's laws being sufficient for certain usual purposes.

(Now when I use the word "rule," I am not implying that the rule came into existence by virtue of the decision of a deity or other entity that that's the way things were going to be. How these rules came into existence I do not presume to know or to have any valuable ideas about. We are used to the idea of things being "made," and of those things therefore having a "maker," but this way of thinking is just something that we are used to doing. The fact that things have a tendency always to happen in a certain way does not logically imply necessarily that someone or something has caused that tendency to exist. And even if someone or something, a maker, did indeed make it that way, we would want to know why, and why there was a maker, etc., so we would still be without a final explanation. So I am sticking only to talking about what we actually find that tends predictably to happen under given circumstances, i.e., certain kinds of situations.)

There is another thing to be clear about regarding the concept of "cause." Even though we have been talking about the very most basic rules of the universe with regard to what will happen in situations, with those rules perhaps or even probably having a great amount of simplicity, it seems most likely that any particular example of a situation causing a particular outcome (resulting situation) is one in which many different parts of that situation contribute to the outcome, making it unlikely that we could accurately say that one component of the situation was the only cause of the outcome. Instead, we would say that the outcome was brought about by all the components of the situation, even though we might also be able to say that, in the situation under consideration, one component was the most important of all, and that just that one component alone was enough to make the outcome that we are thinking about come about. But, in general, any one outcome would be the result of many different causes. When we do "controlled" experiments, we are doing things that allow us to disregard other possible causes of an outcome than the one, or ones, we have an interest in.

(And in fact one component of a situation, a component that we were calling a cause of something, might easily be contributing to other outcomes that we were not even considering, and might even be unaware of. Furthermore, a component of a situation might cause something else to happen, which in turn could have a causative effect (within a particular time period) on the first component, called "feedback." This complexity of causal interaction has led to the concepts of "systems theory" and causative "fields.")

A final point to recognize is that what may be predictable is sometimes unpredictability. This appears to be true in quantum mechanics. But the unpredictability nevertheless is lawful, such that results are indeed consistent with statistical predictions. Repeated experiments may demonstrate that the outcome measurements predictably form a bell-shaped probability distribution curve, or, instead, some other predictable probability distribution curve.

It should be noted that the above-mentioned predictions are not expected to be one hundred percent accurate with one hundred percent certainty. This is because situations are generally fairly complex, containing components beyond what we can measure accurately (or even be sufficiently aware of) that contribute to the outcomes (participate in "causing" the outcomes) that we are predicting. We can predict the weather to a certain extent and we can predict what a person may say or do to a certain extent, but we cannot expect our predictions to be completely accurate or certain. We refer to such inaccuracy and uncertainty as, among other things, "experimental error" or "measurement error." (This has nothing to do with the concept of indeterminacy in quantum mechanics.)

So the bottom line of all of this is that usually when we are saying that something is the cause of something else, we are not referring to a 100% precise and predictable relationship between a causative situation and the caused outcome (resulting new situation). Instead, we are generally referring to "tendencies," so that we say that some considered particular situation was caused by its antecedent situation because situations like that antecedent situation have been found to have a tendency to be followed by situations like the one we are considering.

But even though we cannot expect complete accuracy and certainty, our ability to predict, at least to some extent, is absolutely essential to everything we do. If we were completely unable to predict anything, we would have no reason to do anything, because the reason(s) for doing something is/are at least one or more of the predicted outcomes of doing that something. We do what we do in order to bring about certain predicted outcomes. (And if an outcome is different than what was predicted, we often say we have made a mistake. Such mistakes are the result of inaccurate predictions.)

So we need to summarize what we mean by "cause." To say that a particular situation causes a particular outcome is to say that we believe that the rules (or "laws") of the universe are such that, at least theoretically, we could have predicted that outcome by knowing the (relevant) details of that situation and those (relevant) rules. Note again that the concept of "prediction" is absolutely essential to our concept of "causation."

Similarly, we should also be clear as to what we mean by the related concept, "explanation." We first consider the situation or event that we are trying to "explain" as an outcome. Then by "explanation" we usually mean the description of the causative situation and the (at least implied) statement of the relevant rules of the universe, showing that the outcome we are trying to explain is just an example of outcomes made necessary by the relevant rules of the universe operating in certain kinds of situations, of which our causative situation is one.

To use an oversimplified example, we want to explain an apple dropping from a tree to the ground. We say that (of course) the apple dropped to the ground, because the force of gravity pulled hard enough to break the weakened stem, and that this is an example of the fact that all masses attract each other with a force, which, if it is stronger than the forces of attachment holding one of those objects at a distance from another object, will cause that object to break that attachment and "fall" toward the other object. Our explanation is that the falling of this apple was simply an example of this more general fact, involving how the rules of the universe act in certain kinds of situations to produce certain kinds of (predictable) outcomes.

When we explain our behavior, we describe it as an outcome of the situation we were in, including things about our body, personality, and current beliefs and motivations, so that the listener can "understand" (accurately believe) that we behaved that way because people with that body, personality, and beliefs and motivations in that kind of situation predictably do tend to do as we did. Our behavior is just an example of how people like us tend to behave in situations like that. And we acknowledge that we usually have to make these judgments with some degree of uncertainty and lack of precision.

So let us recognize that when we, in our daily lives, "explain" something that happens, we are not generally using the most basic rules of the universe. Instead, we are saying that what we are trying to explain is an example of what generally tends to happen in such situations. And, of course, what seems like a satisfactory explanation to one person may seem like an unsatisfactory explanation to another, in that the other person may believe that the first person has focused on the wrong tendencies for certain things to happen in situations like the one being considered (or has not "taken some other things into consideration"). It is only in the natural sciences (physics, chemistry, astronomy, biology, etc.) that we would strive to understand these tendencies for certain things to happen in certain kinds of situations using more precise and more basic, underlying rules of the universe.

Let's also recognize that there is another use of the word "cause" (and related words), related to a different meaning of "explanation." When I explain why I decided to do something, I may say that the cause of my doing such-and-such was in order to obtain a particular outcome. ("Explain why he did it." "To make himself popular.") The outcome itself, however, really is not a cause of the behavior in the meaning I have used so far. My prediction of the desired outcome might indeed be one of the causes of my behavior, but the outcome itself would not be the cause. This kind of meaning for "explanation" involves the concept of "intention," a concept that is connected to the concept of "free will," a part of the mental world, and will therefore be discussed in greater detail later. But an extension of this line of thinking into the physical world is involved in the idea of "final cause," in which, for instance, the cause of the acorn is the resulting oak tree. This use of the word "cause," when referring to events in the physical world, is somewhat atypical, probably is not very useful, and does not concern us here. But its use when referring to events in the mental world is indeed quite important to this presentation.

Related to this line of thinking, however, is a very frequent alternative use of the word "cause." So far, we have been dealing with situations in which the question being asked was, "What caused X to happen?" But the alternative question that is sometimes asked is, "Who caused X to happen?" The context is usually one in which we are trying to assign praise or blame to an individual or group, but we can also just be trying to understand a sequence of events. If we imagine a person pushing on a door and the door opening, we would very likely say that the person caused the door to open. A statement such as this, if analyzed further for meaning, might be that the person pushed the door, meaning that he or she exerted a force on the door that overcame the resistance of the friction of the hinges, and thus drift back to the kind of question that asks what caused the door to open. Thus, in a sense, the question as to who caused something to happen is "shorthand" for what caused something to happen, that highlights the identity of one component of the causative situation (the "agent") rather than the rules of the universe that were operative in a particular event.