Secular Humanism is the view that says only the material world exists. All events must be explained in merely natural terms. This view is sometimes called naturalism, or materialism, or material monism, or more popularly atheism. Secular humanists reject the idea of God and spirit. They deny that there is anything more to human nature than the body, and therefore believe that physical death is the end of the individual. The "humanism" part of the name means that persons in this group attempt to affirm human life. As such, they often speak against war, violence, and hate. However, it remains for the secular humanist to prove why individual humans should act one way rather than another, especially if an individual wants to act in a destructive way and derives happiness from that behavior.
The shift from Christian theism, to deism (God creates but does not rule), to naturalism occured during the 18th and 19th centuries. The Wars of Religion in Europe (ended 1648), the divisions among Protestants, and the emphasis on the afterlife in religious revivals like the 2nd Great Awakening (early 1800's in America) led some to look for what is universal and valuable in this life. At the beginning such thinkers may have affirmed God the creator, but rejected the idea that God rules in history (deism). However, as God became increasingly irrelevant for explanations of events in this life, naturalism became the de facto if not expressly held belief system. Darwin's theory of the origin of species is a notable example of naturalistic origins stories.
Modern materialism has its roots in empiricism (the belief that all knowledge is from sense data). Since we only experience, or sense, material things, the naturalists concludes that only material things exist. Strange ghost stories and other such tales are more easily explained by appealing to the material word. This reliance on the senses as the highest authority is often an unspoken assumption, and leds to circular arguments from naturalists when they are asked to justify their belief in the material world (for ex. the material world exists because I experience it; my experiences are reliable because they are the product of the material world).
Secular humanists are particularly noted for their stance as non-religious. However, in my blog entry titled "the definition of religion" I argue that secular humanism is itself just as much a religion as instances like Buddhism, Hinduism, Judaism, Christianity, and Islam. Although it presents itself as neutral, it is, like these other religions, making positive claims about what exists and relying on a highest authority to justify these claims. Although it takes a position of being enlightened and free thinking as opposed to the superstitious and naive religions of human history, it is at its basic level another form of fideism (blind belief) in its empiricism and its claims about the material world.
Because naturalism believes that only the material world exists, it is left with either the claim that the material world has existed from eternity, or that the material world came into existence due to an uncaused event. Although some 20th century materialists have argued that the material world is eternal, most materialists today (like Stephen Hawking and Lawrence Krauss) claim that the material universe began to exist a finite amount of time ago, and that it came into being from non-being. Closer inspection of recent books by these two thinkers shows that what they call "non-being," or "nothing" is really gravity (Hawking) or quantum foam (Krauss), neither of which are "nothing."
Naturalism often appeals to science as the highest achievement of humanity and as confirming the beliefs of naturalism, science itself is limited to making claims about the laws of the material world and not about their origins or if there are non-material kinds of beings. The scientific method states how empirical investigation can be systematized in order to arrive at knowledge of the physical laws. However, this same scientific method cannot be used to explain the origin of these laws, nor can it be extended into the past in order to make claims about time and origins because it prescribes present observation. Many naturalists use the scientific method and then assume that only the material world exists and project deductions about how the world must have formed using this assumption. This is a kind of fideism. In an unfortunate way, this sullies the name of science, and is merely a rhetorical device used by naturalists to claim the benefits of modern science as confirmation of the naturalist worldview. The scientific method can equally well work within other worldviews like theism, and perhaps it is only logically consistent within theism.
If an essential quality of matter is change, then it cannot be the case that the material universe has existed from eternity. If the material universe had existed from eternity and were going through the process of change described by entropy, then it would have already reached its final state of entropy. Since it has not reached this state it cannot have existed from eternity. Appeals to multiverses do not alter this problem.
Similarly, if the qualities of thought and the qualities of matter are not reducible one to the other then both thought and matter persist as real but different kinds of beings. Thoughts, which are described as either true or false, and matter, which is described by extension and motion, are different in kind. Consequently, many materialists who desire to be consistent in their materialism deny the reality of thought. Examples are B.F. Skinner and Paul and Patricia Churchland. The self-referential absurdity of arguing in favor of the thought that thoughts are not real is painfully obvious.