Because the slaves could not write, nor would any words of theirs be printed, William Cowper composed this controversial poem. His writing was as an act of conscience because he felt burdened by the guilt for the sin of his countrymen in enslaving Africans. 

• Appeal to reason 

The speaker of Cowper's poem, a Negro slave, asks for the reason one man can have the right to enslave and torture another.

What are England's rights, I ask, 
       Me from my delights to sever, 
Me to torture, me to task ? 

Also in the first stanza, the speaker calls upon the conscience of the slave owner: " Minds are never to be sold."

• Appeal to logic

In the last stanza, the speaker challenges the slave owner, telling him that before he contends that slaves are mere brutes without human feelings, he should find some justification for slave trading other than the black man's color and

Prove that you have human feelings, 
         Ere you proudly question ours

•  Appeal to emotion

In the first stanza, Cowper mentions that white and black men both feel emotion as "affection dwells" in both of them. Further, in the third stanza, the speaker evokes the feeling of guilt that owners should feel:

Think, ye masters iron-hearted, 
           Lolling at your jovial boards, 
Think how many backs have smarted 
           For the sweets your cane affords.

In addition, the speaker uses a "fire and brimstone" approach by saying that the slave owners are being punished for their heinous acts against humanity by the "One who reigns in high" that the white man worships:

Hark! He answers!--Wild tornadoes 
           Strewing yonder sea with wrecks, 
Wasting towns, plantations, meadows, 
           Are the voice with which he speaks.