Examine the first premise. The first premise is the statement listed first. In the example above, it is: "All apples are fruits." Make sure you have a clear understanding of the premise. In this case, the understanding should be that all apples, regardless of name, species or color, are classified as fruit. This is obviously a simplified premise. The premise you are examining may be more difficult to understand and may require some research or pondering.
Examine the second premise. "All fruits grow on trees" is the second premise in this argument. This means that any living thing classified as a fruit grows on trees. Notice that this premise is only making a positive statement about fruit. It does not state that only fruits grow on trees, so if the argument was that since lettuce is not a fruit, it does not grow on trees, is neither proven nor refuted by this argument. Again, make sure you have a clear understanding of the second premise before proceeding.
Evaluate the argument. Determine whether the final argument is valid or invalid based on the two stated premises. The argument in the example above is that all apples grow on trees. Since you know apples are fruits from premise one, and you know all fruits grow on trees from premise two, you can easily deduce that the argument is valid, and that all apples grow on trees. The argument you are evaluating may not be quite as simple, but as long as you have a clear understanding of the premises, you should be able to determine whether the argument is valid.
Determine if the article is invalid. If the final argument in this example was that since lettuce is not a fruit, it does not grow on trees, this would be an invalid argument, because it is impossible to validate based on the two premises. Often in philosophy, arguments are discussed among many people who try to invalidate the arguments to prove a philosopher wrong. Determining why the reasoning failed helps philosophers correct their arguments. In the above example, if premise two were changed to "only fruits grow on trees," than the argument could be validated based on the provided premises. Another way to validate it would be to change premise one to "lettuce is not a fruit."