Empiricism

I believe in evidence. I believe in observation, measurement, and reasoning, confirmed by independent observers. I'll believe anything, no matter how wild and ridiculous, if there is evidence for it. The wilder and more ridiculous something is, however, the firmer and more solid the evidence will have to be.

Empiricism is a philosophical school holding that knowledge can only be (or is primarily) gained from sensory experience. Accordingly, it rejects any (or much) use of a priori reasoning in the gathering and analysis of knowledge. Along with rationalism, it is the fundamental philosophy behind science and the scientific method.

Empiricism does not imply solipsism, or the notion that nothing is provable beyond our immediate ability to experience the world. Rather, empiricism describes a fundamental worldview which builds upon prior knowledge with our new experiences and an intelligent assessment and interpretation of them.

Empiricism vs rationalism
Empiricism is often contrasted with rationalism. This latter philosophy emphasizes the role of reason and logic in discovering the nature of the world. But while empiricism instead emphasizes the use of the senses, these viewpoints are not necessarily contradictory to most people. Their most extreme formulations aside, most people who call themselves rationalists admit that sensory data is also essential, and few empiricists would say that they never engage in logical reasoning.

The most prominent distinguishing characteristic between these two philosophies is that most rationalists will accept certain a priori truths, believing in some manner of innate knowledge or intuition.

Contemporary definitions of empiricism and rationalism are very different from their historical use, which tended to treat them both as synonyms. In fact, this tendency remains in casual conversation, and generally only people who study philosophy with any depth recognize a distinction.

Empiricism in psychology
Empiricism is used as a term of art in psychology for a school of thought derived from the empiricist philosophical tradition (e.g. Francis Bacon, David Hume). Empiricism is associated with a "blank slate" view of the mind and the behaviorist tradition of B.F. Skinner.

Empirical understanding
Do not believe in anything simply because you have heard it. Do not believe in anything simply because it is spoken and rumored by many. Do not believe in anything simply because it is found written in your religious books. Do not believe in anything merely on the authority of your teachers and elders. Do not believe in traditions because they have been handed down for many generations. But after observation and analysis, when you find that anything agrees with reason and is conducive to the good and benefit of one and all, then accept it and live up to it. Empiricism is sometimes criticized as being short-sighted; a common example is to point out that there are forces at work in the universe that we can't sense, such as gravity, so clearly empiricism is inadequate for the task of making sense of our world. This criticism is sometimes further extended to the scientific method, or science as a whole.

This criticism is based in a fundamental misunderstanding of the philosophy of empiricism, however. It presumes that we cannot accumulate knowledge or draw conclusions, two things that are in fact essential to an empirical approach to the world. Gravity was initially discovered through multiple sensory observations (things frequently fall, especially if you are clumsy) when subsequent analysis eliminated other apparent possible factors. Isaac Newton used an empirical approach to conclude his law of universal gravitation.

A simple example
A simpler example might be microorganisms. Since they cannot be seen with the naked eye, we cannot immediately just say they exist. Even if someone wrote a religious book saying that they exist, we cannot accept an a priori pronouncement of fact without knowing for ourselves.

But we do know that some forms of glass can aid our vision. A simple magnifying glass makes small things look bigger, and we can check and be sure it's consistent. We can deduce rules on how it works, and make bigger or more sharply curved lenses.

Thankfully, we don't have to do that, because clever Arabs, Dutch, Greeks, and Italians have steadily deduced such rules and created such lenses for us. It might seem as though this is accepting their a priori assertions, but instead we only accept things they have proven and that others have replicated. Based on our past experiences, we can reasonably conclude that their fundamental conclusions are correct. We don't have to replicate all of past science and discovery, but we do have to know that we could replicate it if we chose to do so. This is an important part of the scientific method, incidentally: repeatability.

Once we have worked out how to extend our physical sense of sight, we can use our microscope to look at tiny things and see little critters alive down there, such as amoebas and bacteria. And if we have reason to doubt what we see, we can conduct tests designed to confirm our observations. If we think the image in the scope is secretly broadcast from somewhere, we can use alcohol to affect the viewed surface, and see whether it causes any change. It will.

In this same way, through incremental steps of observation, recording, and analysis, we can build up an understanding of the universe based on what we can perceive through our senses.