Not only is it possible to make inferences from is to ought, that is, from descriptive premises to evaluative conclusions -- but it is also possible to make inferences from ought to is.
An example of an inference from is to ought:
Pregnancy is not a disturbance of natural function, but is itself a natural function. Hence it is wrong to view it as view it as a disease which can be "treated" by abortion.
An example of an inference from ought to is:
Lying is wrong. This precept discloses to us that the social practice of conversation is ordered to the mutual discovery of truth -- to a cooperative endeavor to bring thought into alignment with how things really are.
Tomorrow: Of Acorns and Men