The anthropic principle is a group of principles attempting to determine how statistically probable our observations of the universe are, given that we could only exist in a particular type of universe to start with. In other words, scientific observation of the universe would not even be possible if the laws of the universe had been incompatible with the development of sentient life. It supposedly explains why this universe has the age and the fundamental physical constants necessary to accommodate conscious life, since if either had been different, we would not be around to make observations in the first place.
Physical parameters of the universe appear to be fine tuned to permit life forms to evolve. For example, we owe our very lives to the element carbon, which was first manufactured in stars before the Earth formed. The nuclear reactions that facilitate the production of carbon have the appearance of being “just right” to facilitate carbon production. If all of the stars in the universe were heavier than three solar masses, they would live for only about 500 million years, and multicellular life would not have time to evolve. If the rate of the universe’s expansion one second after the Big Bang had been smaller by even one part in a hundred thousand million million, the universe would have collapsed before reaching its present size. On the other hand, the universe might have expanded so rapidly that protons and electrons never united to make hydrogen atoms. An extremely small change in the strength of gravity or of the nuclear weak force could prevent advanced life forms from evolving. An infinite number of random (non-designed) universes could exist, ours being just one that permits carbon-based life.
The strong anthropic principle (SAP) states that the universe is in some sense compelled to eventually have conscious and sapient life emerge within it.
The weak anthropic principle (WAP) states that the universe's fine tuning is the result of selection bias, which is that only in a universe capable of eventually supporting life will there be living beings capable of observing and reflecting on the matter.
The anthropic principle is an argument against intelligent design and creationism which says that some sort of God created all life on Earth and elsewhere. Richard Dawkins sums this up: "You can't use the intelligent designer to explain anything because you have to explain where the intelligent designer came from in the first place. The beauty of evolution is that it starts with simplicity and works up to complexity, to the illusion of design."
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