Tenet #1: We live in a rational, knowable, physical universe. Effects have natural causes. No supernatural events have ever been unquestionably documented.
A lot of scientists and atheists that visit this site will surely look at this and say...of course! And that's why it's tenet number one. It's the first building block of a philosophy because it concerns what it is we can actually know. In this case, it says we can know our universe. That sounds like a lot, but this statement rules out plenty of things that too many people still wish existed. Ghosts, interventionist gods, communicative souls, paranormal activity, miracles, the power of prayer, ESP, fortune telling, spirit mediums, past lives, guardian angels, tempting devils, mischievous trolls, vengeful ancestors, horoscopes, mysterious life forces or energy flows such as qi, prana, or the Force, superstitions, lucky idols, breatherism, voodoo. The list goes on and on. And the amount of time, energy, money, and disagreement that goes into them are a terrible waste.
In billions of years of existence, none of the 100 billion people who have ever lived on this planet have ever captured proof of any of these supernatural items. And that's a good thing! Imagine a universe where seas did magically part, people did randomly come back to life, fishes and loaves did miraculously multiply. Imagine just as many evil magical acts occurring on a regular basis. In a world like this, no science would ever be possible. Repeatability of experiments would be for nought. We could never know anything for certain and so would therefore never learn what we need to know to survive. We would be helpless in such a world!
But we do not find ourselves in this kind of world. We find ourselves in a rational, knowable, physical universe. We can determine the causes of effects we see. And for this we should be glad. For this, we should happily put away the supernatural explanations our less-informed ancestors have passed on to us. We should evolve our philosophy to reflect the actual world we live in, which we continue to discover all the time through scientific explorations. When we evolve our philosophy as a result of what we learn, we become happier as we become better suited for survival. Magical thinking may have comforting benefits in the short term, but it cannot match the long term potential of rational thought, simply because it does not reflect the rational world we live in. All evidence points us this way, so it's the best place to start our journey.