J.I. Packer's definition of "sin"

I've had Keeping in Step with the Spirit by J.I. Packer on my shelf for quite some time.  Every time I walk past the book I say, "yes, I will get to that!"  Well, I've finally got to it.  The purpose of Packer's book is to raise awareness of the presence and work of the Holy Spirit in the spiritual formation of Christians.  Without getting into too much detail (this is not a book review), I wanted to focus in on a quote which I think is helpful in understanding the Spirit's role of making Christians aware of sin - a definition of sin itself.  The word and even concept of sinfulness is used quite a lot, but I struggle with a precise definition (I would even go as far to say that in our cultural setting we need a new more relevant description that people can understanding).  All that being said, I think Packer provides us (me) with a helpful definition, 

"Sin, which is in essence an irrational energy of rebellion against God - a lawless habit of self-willed arrogance, moral, and spiritual, expressing itself in egoism of all sorts . . . " (34).