What the Fallacy?!
Composition/Division
The Fallacy
Assuming that what's true about one part of something has to be applied to all, or other, parts of it.
Why it's wrong
Often when something is true for the part it does also apply to the whole, or vice versa, but the crucial difference is whether there exists good evidence to show that this is the case.
Example
Since the individual atoms that make up this brick are invisible to the naked eye, the brick itself must also be invisible. Unfortunately, David stubbed his toe on reality.