Possible Duplicate:
Where did prefix exceptions originate?
- efficient
- accessible
- consistent
- articulate
- considerate
- conceivable
- convenient
- inefficient
- inaccessible
- inconsistent
- inarticulate
- inconsiderate
- inconceivable
- inconvenient
However valuable to invaluable sits alone, like a trap, not merely an inconvenient irregularity but seemingly almost inconceivable that it's not a deliberate inconsistency, causing an inconsiderate degree of inaccessibility for the inarticulate. So English is even more inefficient and inaccessible than it would otherwise be.
Or is there a logical reason why invaluable doesn't mean unvaluable?