In reply to Ed Ham:
Drawing the line between positive and negative testing is very easy if you have a specification of design requirements. Positive testing tests that the design does what the document says it should do. Negative testing tests that a design can handle stimulus that is outside the requirements and recover in a graceful way. There are many resources on the internet regarding this topic, especially for software design. Negative testing in software usually involves a human being erroneously providing stimulus that is outside the specification. That stimulus space could be boundless. But there has to be be some practical restrictions on what is physically possible or some set of assumptions as what could be provided as stimulus to make the problem manageable. This is probably more true with hardware design than software.
In practical terms, negative testing can be thought of finding what is missing the design spec, getting the spec updated, and turning those tests into the positive.