Intelligent Design (ID) asserts that the universe and life forms we see today are best explained by design of an intelligent cause; Some ID proponents do not rule out adaptations of species, but they do not believe the addition of new complex information could have happened naturally. In other words, they believe natural variation within species is possible, but not new species. Both are trying to explain how the world works. Both claim to search for truth. Proponents of each believe they have proposed a theory.
Within science a theory is generally defined as a systematic framework that explains observations and experimental results, and which can be used to make testable predictions which in turn can either be used to refine or falsify the theory. Generally speaking scientists will attempt to test new theories by designing experiments that, if successful, will falsify the theory. All scientific theories and results are considered tentative and subject to revision or refutation as more evidence is gathered.
Scientific explanations for questions about our world and everything in it are formed by using the scientific method. First, a scientist would come up with an explanation to an observed reoccurring pattern in nature, this is called a hypothesis. The hypothesis is tested by gathering more data by observation and experimentation to see if the data supports or falsifies the explanation. If enough data is gathered, the hypothesis can be considered to have been validated and it becomes a theory. But even a theory is still subject to being falsified if enough data is found to prove it wrong. Scientists must publish the results of their experiments, and explain their hypotheses and theories in scientific journals so that other scientists are able to understand their work. It is also important for experiments and theories to be explained in a way that allows other scientists to reproduce the experiments or devise new ones that can either support or falsify the theory. When scientists try to publish their experiments the work must first be peer-reviewed. This is a process where the work they want to be published is reviewed by a selection of other experts in the field (peers) before it is accepted for publication.
The theory of evolution is supported by data which was collected through observation of a wide variety of natural systems and through laboratory experimentation. Most palaeontologists consider the fossil record to provide evidence completely consistent with the theory of evolution and geneticists also consider evidence from the study of genes to also be consistent with evolution. These two separate strands of evidence are also consistent with each other and have been used to successfully make predictions.
Scientists would further argue that Intelligent Design is not a theory because it has no data supporting it and there is no possible way to falsify it. Intelligent Design supporters argue that the evidence to support them is in the inability of current scientific theories to explain how certain features in biological systems came about naturally and that it is impossible for these features to occur naturally so they must therefore be the result of Intelligent Design. Many scientists consider this argument to be invalid and little more than an attempt to insert God, or an Intelligent Designer, into any current gaps in scientific understanding. This is often referred to as the 'God of the gaps' argument. Although the progress of scientific research can constantly fill these gaps in our knowledge this aspect of the Intelligent Design hypothesis could never be falsified because it can always be reapplied where there are still gaps in our understanding.
Some Intelligent design proponents argue that Evolutionary theory is a psuedoscience which lacks any significant empirical evidence to support it whilst others have chosen to argue that Intelligent Design should be taught as a valid alternative to Evolution within science education. Opponents of Intelligent Design argue that it is unsupported by evidence whereas, in their view, the evidence to support Evolution is overwhelming.
The majority of the scientific community have so far rejected Intelligent design as unscientific, amounting to little more than an untested (and some would argue un-testable) hypothesis that has produced no experimental evidence. It should be noted that accepted theories in science can and are overturned when experimental results are published that successfully falsify the prevailing theory.
Evolutionary theory is often criticised for failing to explain the origins of biological life, and that it is a theory that excludes the possibility of a supernatural god, and as such is atheistic in nature. In reality Evolution is explicitly not a theory concerning the origin of life, or the origin of the universe. It assumes the existence of life and is concerned with explaining how these living systems change through successive generations, developing new traits and ultimately creating new species, or as Charles Darwin put it 'the Origin of Species'.
The theory of Evolution within modern science does not explicitly exclude the possibility of an intelligent designer, either as the designer of the first living matter, or of the universe as a whole. Because science is a discipline that attempts to explain observed facts in terms of measurable physical reality it does not allow supernatural entities as part of its explanations or theories because they are incompatible with the scientific method, even if they actually exist. Many scientists have supernatural or religious beliefs but they do not rely on them when using scientific methods to understand the world. Some evolutionary scientists believe in a form of supernatural origin or intelligent design to the universe and that the mechanism of evolution was designed to do precisely what scientists observe it to do.
Each of these proposed theories starts with presuppositions. Evolutionary Theory and other scientific theories begin by assuming that everything we can observe and measure can be explained in terms of natural processes. ID assumes that supernatural design is possible and that its effects can be observed. ID scientists do not challenge the idea of change over time in organisms -just that the addition of certain complex biological information proposed by evolution is impossible without the intervention of an intelligent designer.