1. The first fundamental rule of evidence is that a witness may testify only to matters that are relevant to the case. Irrelevant evidence is not admissible.
2. Relevancy is defined in Rule 401. Relevant evidence is declared admissible, and irrelevant evidence inadmissible, in Rule 402. To be relevant, evidence must have some tendency to prove or disprove one of the issues or disputes that are of consequence to the determination of the case. The key phrase is "any tendency" to help prove something.
3. Whether an item of evidence has any tendency to help prove something is a matter of logic and common sense.
4. Whether an issue or dispute is "of consequence" to the determination of the case is a matter of law related to the elements of the crime or cause of action. For example, whether "specific intent" is a material issue in a criminal case will depend on the statute and also on whether the defendant pleads accident, insanity or alibi as a defense.
5. By defining relevance as "any" minimal tendency to help prove an issue, and then declaring that all relevant evidence is admissible unless another rule specifically excludes it, Rule 402 states a strong policy favoring the admissibility of evidence. If the attorney making an objection has a legitimate argument for exclusion, and the offering party has a legitimate argument for admissibility, Rule 402 favors admissibility.
6. Evidence rarely has absolutely zero relevance. If it did, no sane lawyer would present it. The problem is usually one of pretextual relevancy -- the lawyer is offering evidence with little genuine relevancy for some ulterior reason. For example, if a defendant is charged with possession of child pornography, the prosecutor may offer evidence that the defendant is a former priest who molested a child in 1983. There is arguably some relevancy to this evidence, as there is logically a connection between child pornography and pedophilia, but it's pretty remote. This relevant connection is just a pretext, however. All the prosecutor really wants to do is convince the jury the defendant is a dangerous pedophile who needs to be locked up for the good of the community (even if there is little evidence he knowingly possessed child pornography).
That's where Rule 403 comes into play. It establishes a balancing test for this kind of pretextual evidence. The actual probative value is weighed against the prejudicial effect, and the judge is given discretion to exclude the evidence in extreme cases. The admissibility of most items of evidence objected to as "irrelevant" will actually be determined by Rule 403 balancing. We will take up balancing in the next class.