Whenever the subject of the thermodynamics of irreversible processes TIP is brought up among biologists, there seems to be a very polarized reaction: some have found it a useful tool, others reject it utterly, and for some reason which I do not understand, even fervently. For the present school, Professor Aharon Katchalsky had suggested that we try together to point out some ways in which TIP has been useful, to discuss limitations and shortcomings, and, finally, to show the direction and first results of present efforts, mainly in network thermodynamics1. It would have been my task to show you the darker side of the coin, but now Aharon is not with us to show you the shining side in his unique way, to carry you along with his enthusiasm. I believe it is in Aharon's spirit that I do not attempt to evaluate or commemorate his contributions and achievements, but just talk science and try to convey a little of what was at the center of his interest during the last years.