@profabelmendez That is a strange argument. If plate tectonics is required for intelligent life to emerge, that is already factored into f_i, the factor for the emergence of intelligent life. Although a case can be made that plate tectonics might be required for intelligent life, it is not a given. But the formation of life *must* happen in order for intelligent life to emerge later, so it makes sense that f_l is the factor preceeding f_i.