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One of the most important factors in the equation is $f_l$, the fraction of Earth-like planets in the habitable zone that develop life. It isbasically  a measure of the likelihood of \textbf{life emergence}. Here I won't go into the (interesting) debate about defining what life is; I will use the somewhat restrictive definition "life as we know it", which loosely means anything similar to what we've seen on Earth. Oh well, one has to start somewhere.  How can we estimate the likelihood of life emergence? If life was impossible nobody would know about it. Similarly, the fact that we see life on Earth can not be used to draw conclusions on how widespread is life in the Universe\footnote{If Earth was the only life-hosting planet in the cosmos, we would necessarily have to be living there}.