Ultimately, science is a way of coming to know and understand the natural universe. Using science, we have increased our understanding of nature and made dramatic improvements in the human condition. The power of science resides in its dependence on verifiable observations and its allowance for revision of theories in accordance with new information. The combination of these two components distinguishes science from all other ways of knowing and appreciating the universe that surrounds us. Yet, its dependence upon the objective (that which can be observed in the same way by two or more individuals) also defines the limits of the scientific method.
Many questions lie beyond the scope of science. If a question cannot be phrased as a hypothesis that can potentially be supported or falsified by available objective data, then it is not a scientific question. Consider the following questions.
The first two questions above fall under the heading of "pseudoscience," a term that refers to events or phenomena that have not been verified by the scientific method, but often have many supporters. Examples include unidentified flying objects (UFOs) believed to be spacecraft from other worlds, astrology, extrasensory perception (ESP, or mind reading), reincarnation, and many others.
For a variety of reasons, none of these beliefs can be effectively investigated using the methods of science. In some cases, these beliefs rely on claims of one-time events that cannot be repeated (UFOs). In other cases, no reasonable mechanism is available to explain how something happens; therefore no cause-and-effect test can be devised (astrology, ESP). In all cases, there is no repeatable experiment one could imagine that would disprove the claims to the satisfaction of the believers (Trefil, James, and Robert M. Hazen, The Sciences an Integrated Approach, 2nd ed. [New York: John Wiley & Sons, Inc., 1998], 12-13).
To be verified, the observations made in science have to be repeatable. The problem with question 1, "Have alien spacecraft landed on Earth?" is that it is asks about a non-repeatable event. Although we could watch the skies carefully from now on (and many scientists are), we cannot go back in time to observe a spacecraft someone claims to have seen land.
On the other hand, we can use our scientific knowledge to look for and examine evidence of such an event. Something as large as spacecraft would surely leave an impression where it landed. Perhaps airport radars picked up an unusual signal. Scientists have investigated some claims of UFOs using such methods, but so far no substantiated evidence of alien spacecraft are available to support these claims. The many photos seen in UFO magazines are, by themselves, of no scientific value. There is no way to differentiate these from the special effects seen in science fiction movies. SETI, the Search for Extraterrestrial Intelligence, is an educational project dedicated to the search for scientific evidence of life on (or from) other planets. SETI has dedicated a number of years to the search for signs of extraterrestrial life and, thus far, has not found it.
Question 2 refers to astrology or the zodiac, which derives from an ancient Eastern tradition that supposes that the positions of the stars and planets at the time of a person's birth determine some aspects of personality and character, that person's zodiac sign. One problem with this idea from a scientific view is that no mechanism has been proposed to explain how the positions and the stars might have a bearing on personality formation. Thus, there is no cause-and-effect relationship to be tested. Despite this, one could still test for a correlation between specific personality traits and date of birth. So far, no reasonable evidence to support the notion of astrology exists.
Question number 3 cannot be answered using the scientific method because there is no objective means available to define or to measure God. However, this is not to say that the religious traditions and ideals that enrich many people's lives are incompatible with science. James Trefil and Robert M. Hazen suggest, "There should be no conflict between the questions asked by science and those asked by religion, because they deal with different aspects of life. Conflicts arise when zealots on either side try to apply their methods to questions where those methods aren't applicable." (Trefil and Hazen, 12).
Science is not capable of exploring ethical questions, such as number 4 above. No experiment can determine moral right from wrong, because morality is a value judgment—it is subjectively defined. Science can only address questions that can be answered by considering objective information. Scientists themselves, like any group of people, vary considerably with regard to their moral and ethical beliefs. This is not to say that scientists should not be required to act in an ethical way. In fact, researchers must abide by a number of guidelines developed by committees on ethics to assure that certain standards are upheld.
For some of the same reasons, question 5, "What is the meaning of life?" is beyond the scope of the scientific method. This is not to say that the question is without value, or that an answer should not be sought! But it is a question that would be answered quite differently by different individuals. If your answer and my answer to this question disagree, that does not prove either of our conclusions false.
Beauty, as the saying goes, is in the eye of the beholder. Science cannot be used to measure or compare the aesthetic value of two works of art (question 6). Science cannot help us to enjoy a clever play or to laugh at a joke. Science may explain why the sky is blue, but it cannot tell us why it is beautiful—nor is it meant to.
Fortunately, a variety of disciplines apart from science are dedicated to exploring the many facets of the human experience: literature, philosophy, the Arts. These studies have a long history and occupy an essential niche in human cultures. They lend depth and significance to the human experience and enrich our personal lives. They serve as a great compliment to science, in that they stimulate and encourage development of an entirely different set of human sensibilities and values. They provide an essential counterbalance to the activities of science.
American poet, Walt Whitman, eloquently characterized some of the limits of science in his poem, "When I Heard the Learn'd Astronomer."
When I heard the learn'd astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts and diagrams, to add, divide, and measure them,
When I sitting heard the astronomer where he lectured with much applause in the lecture-room,
How soon unaccountable I became tired and sick,
Till rising and gliding out I wander'd off by myself,
In the mystical moist night-air, and from time to time,
Look'd up in perfect silence at the stars.
(Francis Murphy, ed., Walt Whitman, The Complete Poems [Harmondsworth: Penguin Books Ltd., 1975], 298.)