The model penal code is law nowhere, so your professors love it. It's theoretical - a guiding light of reason that many states follow in enacting their criminal codes.

The MPC defines intentionality as I described in the criminal law section: Purpose, Knowledge, Recklessness, and Negligence. Defining these terms in detail is too tedious for me and for you. You'll spend a great deal of time memorizing the MPC definitions, later.

For our purposes we'll say:

1. Purpose = conscious object, you meant the results of your actions
2. Knowledge = you knew the results that would follow your actions
3. Recklessness = knowledge of a substantial risk that your actions would lead to a certain result
4. Negligence = unreasonable ignorance of the results of your actions.

The MPC also defines crimes and breaks them into their constituent elements. Then it applies the above mentioned levels of intentionality. This is just the old mens rea + actus reus formula.

The common law, i.e. judge made law, also defines crimes and their requisite elements. In a majority of states codes, like the MPC, have been passed, but most jurisdictions explicitly maintain common law crimes by way of a statute that incorporates them into the code.

At common law, the MPC mens reas of Purpose, Knowledge, and recklessness are represented by different names. At common law crimes are sometimes classified as general vs. specific intent crimes. With specific intent crimes the mens rea required is something akin to the MPC's 'purpose.' General intent crimes have a lesser mens rea.

For more about codes see the codes section of the site.