Scientific theories should require the ability to be falsified.

!. Falsifiability is necessary for a theory to be usefulScience is devoted to the development of theories that make useful predictions. If a theory agrees with everything that can possibly be observed, it has no practical utility. Practical utility means support for technology. Knowing laws of nature allows constructing bridges that hold up to use, integrated circuits that work in computers, drugs that cure disease, and that accomplish all the physical tasks that support life. Let's momentarily suppose that all scientific theories make predictions. For example, Newton's Law says that force is equal to mass times acceleration. That predicts that if we know two of the three quantities of force, mass, and acceleration, then the third is accurately predicted by the formula. The theory can then be falsified by comparing the predicted value of the quantity with what is observed. Any theory that enables making useful predictions can therefore be falsified. Now consider a theory in which any observed outcome is consistent with the theory. Such a theory cannot be falsified because nothing that happens is contrary to the theory. Consider the theory, “All events in the world occur according to a grand plan that cannot be known.” Anything that happens is consistent with that theory, and therefore provides no help in building bridges or anything else. A theory that cannot be falsified makes no useful predictions. Making any prediction with practical consequences allows the possibility that the prediction will be wrong, and the theory is thereby falsified. People are free to speculate about unprovable theories, but that's an activity outside of science. 2. Falsification is necessary for a theory to have explanatory power Someone put it as, "A theory that explains everything explains nothing." A scientific explanation relates causes to effects: D happened because of A, B, and C. Saying that D happened because of a theory, but anything else might have happened as well, does not explain D. It goes no further than saying that D was observed. A falsifiable theory not only allows D, but it disallows other outcomes. If the not-predicted outcomes are observed, then the theory is invalid.Because non-falsifiable cannot be contradicted, an infinite number of non-falsifiable theories can coexist. For example, causes might be attributed to invisible spirits of various kinds arranged in various hierarchies. A methodology that allows an infinite number of alternative explanations has little explanatory power. At some point there may be more than one valid scientific theory that explain an event, but choices are severely limited and falsifiability provides the possibility of determining which is true.Science strives not only to provide useful theories but to explain how the universe works. Falsification is necessary for explanation.3. Scientific enterprise is distinct from a scientific theoryData collection is a part of the scientific enterprise. A geologist may assemble a map showing, say, the rock and soil types of New Mexico. That's a scientific activity that doesn't involve falsifiability. If the geologist forms time line theory from the data, that's subject to testing.Scientists are also free to speculate as part of the scientific enterprise. They can think of something before considering whether or not it can be formulated as a valid scientific hypothesis or theory. If it cannot be formulated as scientific, they can either abandon the idea or try to modify it so it valid. However, the work des not join the body of legitimate science until it is shown to be falsifiable.For example, string theory is an attempt to link the fundamental forces of nature into a coherent common structure. The hypothesized structure is then considered as a potential scientific theory. There is controversy over whether string theory is a valid scientific theory or not. [3. http://www.newyorker.com...] Whether it now qualifies or not, scientists, including advocates, agree it must be falsifiable. Activities leading up to a valid scientific theory need not be proved falsifiable at every point, but the final product must be.4. Falsification is not the only consideration of science, but it is criticalString theory brings up the point that falsification only gets rid of theories, it doesn't prove them uniquely correct. “When string theory was proposed, it was used to predict flux tubes of strong force. It has also predicted black hole entropy....The problem in both cases are that more simple theories have done the same. So it has passed testing, but not unequivocal testing. That is, you have to complement with comparison between theories because you have competitors for the same predictions.” [4. http://whyevolutionistrue.wordpress.com...]But that's a universal problem in science, not a reason to abandon falsification as a criteria. No scientific theory is ever proved to be universally true. The condition that that a theory is falsifiable is a necessary condition for it to be scientific, but it is not the only consideration at work in science. For example, simple theories are preferred over complex ones. Symmetry is often demanded in a theory, i.e., that it have no preferred direction in space. By a “valid scientific theory” we mean a theory that qualifies as having the potential to be a scientific theory. If the test is performed and fails, then it isn't a valid description of nature, but it was a valid candidate. 5. A history of bad science does not justify bad science My opponent observed that science has been going on for a long time, but that falsification as a criteria dates to the 50s. Falsification has always been recognized as a criteria. Archimedes discovered that the volume of an object could be measured by the volume of water it displaced. If the theory did not work, would it have persisted? No, falsification has always played a definitive role in science. More recently, "the Michelson–Morley experiment was performed ... to detect the relative motion of matter through the stationary luminiferous aether ("aether wind"). The negative results are generally considered to be the first strong evidence against the then prevalent aether theory" [5. http://en.wikipedia.org...] There has also been bad science for a very long time. Astrology persisted for a very long time, because all events were found to have astrological explanations. In the 19th century, phrenology, the "science" of predicting character from the shape of person's head, was widely accepted. what past bad science shows is that sharper methods of distinguishing bad science were needed. Popper pointed out that the strength of a theory is in proportion to it's ability to be falsified. General relativity predicted that light would be bent by gravity. That's such an extraordinary and definitive prediction, that when the bending was observed in a solar eclipse it provided substantial proof of the theory. That was one of the events that inspired Popper's observations. If A is proportional to B, there is a relationship such that A equals k, the constant of proportionality, time B. Hence if the ability to be falsified is zero, the strength of the theory is zero. The Stanford Philosophy article Con cited says, “... factors combined to make Popper take falsifiability as his criterion for demarcating science from non-science: if a theory is incompatible with possible empirical observations it is scientific; conversely, a theory which is compatible with all such observations, either because, as in the case of Marxism, it has been modified solely to accommodate such observations, or because, as in the case of psychoanalytic theories, it is consistent with all possible observations, is unscientific.” [6. http://plato.stanford.edu...]