Gemara: How are men checked (i.e., in what type of situation do their sins "catch up with them")? Reish Lakish says: When they pass over a bridge. Only a bridge? In other words, cases like a bridge.
Ein Ayah: [The following is a quite difficult and esoteric piece, which we have tried to explain as well as we can.]
Nature and melacha (that which man does) together determine a person’s life, but, of the two, it is nature that is the foundation, andmelacha is built on top of it. As long as man is in "the hands of nature," he is safe from failure, as the Creator made the world in a manner that nature will protect those who fall in line properly.
Man is beyond the point at which he can rely only on nature, and he has to complement it with melacha, both in the material and in the spiritual realms. This world that man augmented will be successful to the extent that the individual has a strong natural base. Then, he will not take the damaging step of trying to destroy his nature. If, though, he tries to fight the positive parts of his nature and replace it with something artificial, then he is in danger of the entire structure collapsing.
A person does not live forever with only the nature with which Hashem created him. However, passing from one form to another can be artificial, and enabling the transfer is a proper use of melacha. When a person has a strong natural base that connects well with his melacha, it will be easier to make the transfer. If he has a weak base, then he will be vulnerable, and everything might collapse during the transfer.
The idea of a bridge over water is an excellent metaphor for what a person has to survive when going from one state of grounded area (i.e., stability) to another, with an area of "water" in the midst serving as an impediment. There are many such places of passage, in which a person and his acquisitions go through a change. Sometimes the person has a strong connection with both the previous and the subsequent stage. There are other times when the transfer is between areas in which there does not seem to be a possibility of passage. It is still possible, though, if the melacha can connect itself to the nature and if one is able to truly recognize that the melacha is a form that the nature, which Hashem provided, takes.
These phenomena occur both in the physical world, such as the state of his body, and the spiritual world, such as a person’s beliefs, practices, and aspirations. It is critical for the person to recognize the way of Hashem to the degree possible, and it must come along with a moral conviction that is consistent with the morality of the world. The power to proceed is strengthened within his nature when the person has followed the path of Torah and has been nourished by the wisdom of the Torah.
This is the deeper level of what the gemara means that men are checked when they pass over bridges or things that resemble bridges. Passage from stage to stage is dangerous, physically and spiritually, and one must be careful in the process. The most important thing is that one must be prepared to take the step, including the step in the upward direction, and know how to remove the stumbling blocks along the way. Hashem’s eyes are everywhere in the world, and therefore, man has to be worthy of His help in order to succeed.