The cause-effect connection can be conceived as a one-way, one-directional action only in the simplest and most limited cases. The idea of causality as the influence of one thing on another is applied in fields of knowledge where it is possible and necessary to ignore feedback and actually measure the quantitative effect achieved by the cause. Such a situation is mostly characteristic of mechanical causality. For example, the cause of a stone falling to the ground is mutual gravitation, which obeys the law of universal gravitation, and the actual fall of the stone to the ground results from gravitational interaction. However, since the mass of the stone is infinitely small compared with the mass of the earth, one can ignore the stone's effect on the earth. So ultimately we come to the notion of a one-way effect with only one body (the earth) operating as the active element, while the other (the stone) is passive. In most cases, however, such an approach does not work because things are not inert, but charged with internal activity. Therefore, in experiencing effect they in their turn act on their cause and the resulting action is not one-way but an interaction.
In complex cases one cannot ignore the feedback of the vehicle of the action on other interacting bodies. For example, in the chemical interaction of two substances it is impossible to separate the active and passive sides. This is even more true of the transformation of elementary particles. Thus the formation of molecules of water cannot be conceived as the result of a one-way effect of oxygen on hydrogen or vice versa. It results from the interaction of two atoms of hydrogen and one of oxygen. Mental processes are also a result of the interaction of the environment and the cortex.
To sum up, all processes in the world are evoked not by a one-way or one-sided action but are based on the relationship of at least two interacting objects.
Just as various paths may lead to one and the same place, so various causes lead to one and the same effect. And one and the same cause may have different consequences. A cause does not always operate in the same way, because its result depends not only on its own essence but also on the character of the phenomenon it influences. Thus, the heat of the sun dries out canvas, evokes extremely complex processes of biosynthesis in plants, etc. Intense heat melts wax but tempers steel. At the same time an effect in the form of heat may be the result of various causes: sun rays, friction, a mechanical blow, chemical reaction, electricity, disintegration of an atom, and so on. He would be a bad doctor who did not know that the same diseases may be due to different causes. Headache, for instance, has more than one hundred.
The rule of only one cause for one effect holds good only in elementary cases with causes and effects that cannot be further analysed. In real life there are no phenomena that have only one cause and have not been affected by secondary causes. Otherwise we should be living in a world of pure necessity, ruled by destiny alone.
To understand the cause that engenders a change in the state of an object we should, strictly speaking, analyse the interaction of the object with all other objects surrounding it. But experience shows that not all these interactions are equally significant in changing the state of the object. Some are decisive while others are insignificant. So, in practice, we are able to single out a finite number of decisive interactions and distinguish them from those that are secondary.