The knife is the oldest known implement, used as a cutting tool at the dawn of Mankind.  

The knife as an eating utensil didn't develop until the Middle Ages.  At that time, peasants would carry a
knife with them at all times, having the knife do double duty as both an eating tool and a weapon.  Noblemen
would carry two knives, one for each purpose.  It was common for a whetstone to be placed just outside the
entrance of a great hall so that guests could sharpen their knives before a feast.  From this we get the
expression "to whet the appetite."  During the meal, knife blades were used for cutting and the tips of the
knives were used to spear meat and used in the manner we use forks today.  Blades were used to pick up
smaller items that weren't feasible to be eaten with the hands, such as peas.  So in its early days, the knife
took on the role of knife, fork, and spoon all at once.

It wasn't until the 16th century in Italy that the true dinner knife emerged, one that's use was strictly for
eating.  Even then, only the wealthy could afford to supply their guests with them.  

Louis XIV was the first king to provide each guest with a knife, fork and spoon.  By the 18th century it had
become fashionable for wealthy people to have sets of matched flatware.