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$N \approx f_l \times f_i \times f_c \times L$  The first interesting term to analyse is $f_l$. Given the presence of the right conditions, how likely is life to emerge?   The problem is that if life was impossible noone nobody  would know it and at the same time our existence on Earth can not be used to draw conclusions on how probable is life in the Universe. Even if this probability was extremely low and life existed only on one planet in the Universe we would necessarily be on that planet. %Having one data point, the Earth, doesn't give any statistical information: the solar system could well be an extremely %unlikely, or even unique, place in the Universe. As soon as you ask the question "Are we alone?" it means you are alive %and conscious, i.e. you are in a sweet-spot, and you can not make any claim about how common that might be. Beside the fact that one such place exists.