Atheism, as a philosophical view, is the position that either affirms the nonexistence of gods or rejects theism. When defined more broadly, atheism is the absence of belief in deities, alternatively called nontheism. Although atheism is often equated with irreligion, some religious philosophies, such as secular theology and some varieties of Buddhism such as Theravada, either do not include belief in a personal god as a tenet of the religion, or actively teach nontheism.

In biology, evolution is a change in the inherited traits of a population from one generation to the next. These traits are the expression of genes that are copied and passed on to offspring during reproduction. Mutations in these genes can produce new or altered traits, resulting in heritable differences between organisms. New traits can also come from transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. Evolution occurs when these heritable differences become more common or rare in a population, either non-randomly through natural selection or randomly through genetic drift.

Homeopathy is a controversial form of complementary and alternative medicine created in the late 18th century by German physician Samuel Hahnemann. The principles of homeopathy were laid out in his textbook, The Organon of the Healing Art, which remains in wide use today.

Iridology (also known as iridodiagnosis) is an alternative medicine technique whose proponents believe that patterns, colors, and other characteristics of the iris can be examined to determine information about a patient's systemic health. Practitioners match their observations to iris charts which divide the iris into zones which they correspond to specific parts of the human body. Iridologists see the eyes as "windows" into the body's state of health.

Parthenogenesis is an asexual form of reproduction found in females where growth and development of an embryo or seed occurs without fertilization by males. The offspring produced by parthenogenesis are always female in species where the XY chromosome system determines gender.

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as based on similarity of DNA or morphology. Presence of specific locally-adapted traits may further subdivide species into subspecies.

The word theory has a number of distinct meanings in different fields of knowledge, depending on their methodologies and the context of discussion.

In science, a theory is a mathematical or logical explanation, or a testable model of the manner of interaction of a set of natural phenomena, capable of predicting future occurrences or observations of the same kind, and capable of being tested through experiment or otherwise falsified through empirical observation. It follows from this that for scientists "theory" and "fact" do not necessarily stand in opposition. For example, it is a fact that an apple dropped on earth has been observed to fall towards the center of the planet, and the theories commonly used to describe and explain this behaviour are Newton's theory of universal gravitation (see also gravitation), and general relativity.