Can't is the worst word that's written or spoken;  
Doing more harm here than slander and lies;  
On it is many a strong spirit broken,  
And with it many a good purpose dies.  
It springs from the lips of the thoughtless each morning  
And robs us of courage we need through the day:  
It rings in our ears like a timely-sent warning  
And laughs when we falter and fall by the way.  
 
Can't is the father of feeble endeavor,  
The parent of terror and half-hearted work;  
It weakens the efforts of artisans clever,  
And makes of the toiler an indolent shirk.  
It poisons the soul of the man with a vision,  
It stifles in infancy many a plan;  
It greets honest toiling with open derision  
And mocks at the hopes and the dreams of a man.  
 
Can't is a word none should speak without blushing;  
To utter it should be a symbol of shame;  
Ambition and courage it daily is crushing;  
It blights a man's purpose and shortens his aim.  
Despise it with all of your hatred of error;  
Refuse it the lodgment it seeks in your brain;  
Arm against it as a creature of terror,  
And all that you dream of you some day shall gain.  
 
 
Can't is the word that is foe to ambition,  
An enemy ambushed to shatter your will;  
Its prey is forever the man with a mission  
And bows but to courage and patience and skill.  
Hate it, with hatred that's deep and undying,  
For once it is welcomed 'twill break any man;  
Whatever the goal you are seeking, keep trying  
And answer this demon by saying: 'I can.'
Edgar Albert Guest
http://www.poemhunter.com/poem/can-t-13/