For a good to be classed as a public good it must fit into two categories, these being:
- Non excludable - This means that individuals cannot be excluded from consuming the good. Using the street lights example, it's virtually impossible to stop people consuming them once they have been provided, thus they can be classed as non-excludable.
- Non rival - This means that consumption by one individual does not affect the consumption of others. With street lights, if one person is using the light it isnt stopping others using it as well, thus they are non rival as well.