And this equation basically gives us the conditions under which a reaction is spontaneous and under which the reaction is not spontaneous. So let's call this equation number one, and let's recall what the definition of entropy is for constant temperature. Now, the reason we're going to assume constant temperature is because all the different types of biological reactions that take place in nature usually take place under constant temperature conditions. So, for example, reactions that take place inside our body and inside the cells of our body take place at a constant temperature of about 36.8 degrees Celsius. So let's suppose we're inside our cell, and inside that cell, some type of intracellular process takes place. So our cell and the process is a system, and everything outside the cell are the surroundings.