As opposed to religions, which are founded on unsubstantiated if not outright debunked ideas, Secular Humanism is a philosophy of life founded on skepticism, empiricism, and reason. As a result, we not only have the capacity to better understand what is likely to be true, but we can also discern what is probably false, including supernaturalism.
Humans are not always the best judges of what is real/true and what isn't. Our brains are not video recorders that keep perfect records of past events. Instead, memories become distorted and change over time. In addition, we tend to view the world through the lens of our own biases, preconceived notions, emotional states, desires, etc, which skews our understanding of reality. As I mentioned in previous posts, when people hold strong beliefs, they will go to almost any length to maintain them. In addition, we also hold many unrealistic beliefs about our abilities and the probability of positive events occurring in the future. On top of all of this, we are susceptible to visual and auditory hallucinations and illusions, which impact our perceptions of the world. Finally, people lie to manipulate others to suit their own agendas. Thus, it pays to have a healthy amount of skepticism toward the stated beliefs of others as well as our own perceptions of reality. Sometimes what is obviously true is actually false, and what is obviously false may actually be true.
Since even our own feelings, judgments, and memories may be faulty, the only way to be able to ascertain probable truth is the availability of quality evidence. The best evidence derives from science, because scientists are trained in research methods and statistical techniques that have been shown to be effective in uncovering the probability of patterns. We know science works because, as Richard Dawkins so eloquently put it, "Planes fly. Cars drive. Computers compute. If you base medicine on science, you cure people. If you base the design of planes on science, they fly. If you base the design of rockets on science, they reach the moon. It works, bitches." Thus, one good study conducted by a good scientist can help to understand what is probably (i.e. more than 50% likely to be) true. However, other confirming studies and/or a consensus from the scientific establishment are required to be confident that any particular explanation is very probably true.
Most of us do not have the luxury of a scientific consensus for every aspect of our lives in which we need to form judgments. In addition, sometimes we alone need to review the best available evidence and come to a conclusion regarding what the evidence suggests is probably true. This requires the ability to reason. There are three main types of reason:
Deductive: (conclusion guaranteed) starts with the assertion of a general rule and proceeds to a guaranteed specific conclusion. E.g. if x = 4 and y =1, then 2x + y = 9
Inductive: (conclusion likely) begins with observations that are specific and limited in scope, and proceeds to a generalized conclusion that is likely, but not certain, in light of accumulated evidence. E.g. all cats that you have observed purr. Therefore, every cat purrs.
Abductive: (educated guess) begins with an incomplete set of observations and proceeds to the likeliest possible explanation for the set. E.g. the lawn is wet, so it probably rained last night.
Fallacies are errors in logic, which invalidate any argument for a particular claim. While fallacies may show a line of reasoning is false, they do not necessarily indicate the underlying claim is false. However, untrue claims are more likely to be believed by those who employ flawed reasoning to support them. Thus, while fallacies don't invalidate claims, they are a strong red flag that one should be skeptical of the claims. Given their usefulness, it helps to know the main types that people employ during arguments. Here are a few:
- Appeal to Nature: because something is 'natural' it is therefore valid, justified, inevitable, good or ideal. 
- Ad Hominem: an attack on an opponent's character or personal traits in an attempt to undermine their argument. 
- Argument from Ignorance: claiming that something is true because it has never been proven false, or that something is false because it has never been proven to be true. This is often phrased as "absence of evidence is not evidence of absence."
- Appeal to Tradition: occurs when it is assumed that something is better or correct simply because it is older, traditional, or "always has been done."
Beyond fallacies, there are other red flags that tend to pop up when claims are likely to be false. For example, the Richard Dawkins Foundation for Reason and Science and Skeptic Magazine editor Michael Shermer worked together to form a 10-point checklist for assessing the believability of a claim as a part of their "Baloney Detection Kit". While it may not be exhaustive, it is certainly useful:
- How reliable is the source of the claim?
- Does the source make similar claims?
- Have the claims been verified by somebody else?
- Does this fit with the way the world works?
- Has anyone tried to disprove the claim?
- Where does the preponderance of evidence point?
- Is the claimant playing by the rules of science?
- Is the claimant providing positive evidence?
- Does the new theory account for as many phenomena as the old theory?
- Are personal beliefs driving the claim?
In order to discern quality evidence and arguments, one must be educated. It requires mental training to think critically and to properly analyze data and arguments. It takes knowledge of the world and of recent scientific advances in order to effectively apply these skills to make quality life decisions. While education from school certainly provides much of the foundation for these skills and knowledge, they fade over time without use. In addition, without continually seeking new knowledge, people may make judgments based on old ideas that have been shown to be untrue. Thus, good Secular Humanists not only value reason and learning, but we also accept that probable truths change as more scientific knowledge is accumulated.
Given our predilection for reason, evidence, and education, Secular Humanists do not believe in any gods or supernatural forces. There is simply no evidence for the supernatural that cannot be better explained by known natural phenomena. Unfortunately, given our innate inclinations to believe in invisible agents and magical forces, it tends to require a lot of reasoning skills and intelligence to acknowledge this fact. This is strongly supported by scientific evidence, as those who have a greater capacity to override their intuitive cognition (a.k.a. "rational" thinkers) tend to be less likely to be religious. This would partially explain why, in a 2009 Pew survey, 2/5 of American scientists said they did not believe in a god or a higher power, compared to 4% of the general public. Beyond rational vs. intuitive thinking, general intelligence is also a strong indicator of irreligiosity. In a recent meta-study of 63 separate studies, 53 showed a strong negative correlation between intelligence and religiosity. As the study concluded "Most extant explanations (of a negative relation) share one central theme - the premise that religious beliefs are irrational, not anchored in science, not testable and, therefore, unappealing to intelligent people who 'know better'."
Secular Humanists understand that humans are flawed thinkers. Thus, we seek to rely on the best available explanations of the best available evidence to understand what claims are likely to be true. Since there is no clear cut set of rules regarding what will be true for all time, we strive to equip ourselves with the tools of reason and knowledge to navigate life. As a result of our strict desire to understand the truth, we reject the belief that gods and other supernatural forces probably exist.
Funny article regarding the 5 psychological flaws that alter our perception of reality
Fun logical fallacy resource
Baloney detection toolkit video and list
Good article about how rational cognition leads to irreligiosity