The act of reasoning that starts with a given set premises and draws a conclusion is called deductive reasoning.
Deductive reasoning begins with general concepts and principles and draws specific conclusion by starting with a given set premises and draws conclusions from them. The conclusions made by way of deductive reasoning are only as valid as the premises on which they are based such that only one false premise can produce a false conclusion. A valid deductive argument t is one where its truth necessarily follows from the starting premises. A sound deductive argument is one that is valid and all of its premises are true other wise it is considered unsound.
A common form of deductive reasoning is called a syllogism It has three parts. The first part is a set of general characteristics belonging to a category of objects. The second shows that the object under discussion belongs to that category The conclusion that the object under discussion has that general characteristic.
Deductive reasoning dependents entirely on the validity of premises being used. The premises used may be facts derived from observation or a totally philosophical assumption.
Deductive reasoning is a very useful tool of reasoning however it does have its difficulties. It depends heavily on the accuracy its starting premises. However flawed premises result in flawed conclusions. These difficulties need to be properly understood to properly understand scientific statements including those of both Creationists and Evolutionists.