Why does 'set' syntax not work for syntax highlighting in vim?

On searching how to set syntax highlighting on in vim, I found this page which says that vimrc should have the following line syntax on **Why is it not something like set syntax on?** Like we have set nu, set ai etc. With set syntax on, I get this error $ vi syntax= Error detected while processing /home/lazer/.vimrc: line 30: E518: Unknown option: on Hit ENTER or type command to continue and with set syntax, I get this $ vi syntax= Hit ENTER or type command to continue