[jQuery] Selector Help

How would I go about adding class to the li in this set up?

<li><a href="#" class="filter"><span>all</span></a></li>
<li><a href="#" class="filter"><span>some</span></a></li>
<li><a href="#" class="filter"><span>none</span></a></li>

$('a.filter').click(function(){
$(???).addClass('active');
});

Thanks
Dave