Lust

Lust is when the means and ends get swapped-- hence we speak of the soul being disordered.  For example, lust for money is when money becomes the end, rather than the means to a higher end (feeding your family, or paying for your children's education).  Lust for sex is when the person becomes a means to attain pleasure, rather than the pleasure being a means to unite with the person, who is the end.  We must strive to keep our souls rightly ordered; things are never ends, and people are never means.