A discussion has been going on under "Abiogenesis" concerning how to define life, and on earlier threads we talked about the "intelligence" of cells ... not conscious intelligence like ours, but nevertheless some form of it which enables cells to adapt to changing environments or to create new combinations. An article in The Martlet, a magazine published by my old college at Cambridge, may shed some pertinent light, though I have no idea how much of this will be new to our resident scientists. The article, by Michelle Rigozzi ... working as a theoretician in the Physics of Medicine centre at the Cavendish Laboratory ... is too long to reproduce here, but this is an edited version:-"Through the processes of life, tissues turn over; cells of the small intestine, for example, being renewed every three to six days. Furthermore, in times of injury cells in their billions spring up to repair damaged blood vessels, make new skin, muscle, even bone. The question is, how does each cell know what it should do?-Cells are both blind and deaf, and must find their way around their environment using their senses of touch and smell. With the revolutions of the twentieth century in genetics and biochemistry, the focus has been on 'smell'; that is, on the way cells detect chemical signals such as cytokines. New studies have, however, begun to show that touch is very important as well.-If a stem cell is placed on a gel that is flexible like mucus, it will turn into a neuron-like cell. If the stem cell is placed on a gel ten times stiffer, it turns into a muscle-like cell. Put it on something ten times stiffer again and it will turn into a bone-like cell. Such responses to the flexibility of their environment by cells is known as mechanosensitivity.-Mechanics is not the only factor that dictates stem cell differentiation: different chemical factors can also be used. It has been observed that chemical and mechanical influences compete, and mechanics can win. Take, for example, research done on renewing heart tissue: stem cells injected into scar tissue in a heart did not differentiate into new heart cells; instead, within the stiff scar, they differentiated into bone.-[...] My work looks at active responses of biological matter to the mechanics of the surrounding media. One unifying theme of the group's work is the study of polymers, gels, liquid crystals and other so-called soft matter. Soft matter has distinctive physics. Its intermediate scale (mesoscopic ... between the atomic and macroscopic), and its sensitivity to thermal noise give it rich, unusual behaviour, such as self-assembly. The relevance here is that much living matter can also be classed as soft matter, and so shares the same physics.-[...] In cells, the soft matter interacts with signalling; this is a bridge between two pillars of the past century of Cambridge science: Physics and Molecular Biology. Mechanosensitivity is a bridge between soft matter, which organizes through thermal noise, and living matter that organizes through genetically inherited information, with specialized molecules and specific interactions.-Mechanosensitivity has been found to affect huge numbers of different cell behaviours, from cell morphology to cell migration, protein synthesis, proliferation and cell death.-[The author goes on to describe the model she uses.] This collective cell model resembles that describing a passive fluid, but with extra terms that capture the contribution of the vital properties of cells. This raises a question that I think physicists find difficult to answer: what is life? Could life be defined as the extra terms in these equations? When does passive matter become living matter? If a living system is pared down, what is the minimum that needs to be kept for it to be living? The answers may depend on whether the bridge is being crossed from Physics to Biology, or vice versa. Is conceptual thinking adequate to answer these questions, or do we need aesthetics? The questions change again when looking to applications: in diagnostics, drug development, or tissue engineering. To use this knowledge in medicine, how can physics best help? It is hoped the views from these bridges will become clearer as soft matter physics begins to address mechanosensitivity."