Lewis's first appeal to emotion is made in the effective diction of Lewis's preface, "Readers are advised to remember that the devil is a liar," (6; preface) The connotation of the term devils is distinctly evil, thus Lewis gives us a prejudice toward them. Lewis furthers this prejudice with his use of the term liar, indicating that the devil is not to be trusted.
Lewis's second appeal to emotion comes in his characterization of the devils as prideful. In the 31st chapter Screwtape expresses his victory over Wormwood "I have always desired you, as you (pitiful fool) desired me. The difference is I am the stronger," (145; ch.31) through this Lewis shows us one form of Screwtape's pride. This pride, the pride of one looking down on another, leads us to further distrust Screwtape.
It is easy to see the pride of one looking down on another, thinking himself better, but Lewis doesn't confine the devils to a single form of pride. In the 1st chapter he introduces a more complex form of pride, "...oh, that abominable advantage of the Enemy's!"(8; ch.1) Here is shown the pride of the weaker or lower resenting the greater for his strength, and making excuses for his own weakness or failures. This pride shows us another shortcoming of the devils, thus Lewis deepens our distrust of them.
A second characteristic, which Lewis reveals in Wormwood's attempt to send the "Secret Police" after his uncle and mentor Screwtape as displayed in the 22nd chapter is disloyalty. This quality is echoed by Screwtape, "Love you? Why, yes. As dainty a morsel as ever I grew fat on." (145; ch.31) he further explains that because Wormwood had failed he, Screwtape, would get to in some way consume Wormwood. Screwtape displays his disloyalty as he is not upset at his nephew's failure, nor interested in his defence, but rather pleased at the prospect of gaining from Wormwood's punishment. Thus Lewis continues to build our feelings of the evils of the devils right through the final chapter.
Through the words of Screwtape, Lewis appealing to our logic explains the immediate goal of the devils, "the only thing that matters is the extent to which you separate the man from the Enemy." (56; ch.12) One such example is given in the first chapter as Screwtape relates a story of one of his "patients" who he almost lost. This patient, prompted by God, had begun to think religiously, and instead of arguing Screwtape just convinced the man that he needed to go for lunch, thus breaking the line of thought(9;ch.1). Lewis uses this as an appeal to logic, leaving us to conclude that this separation is to our detriment, as it is what our enemy, the devils, desires. This is how Lewis continues to build a foundation for the views he would have us develop.
Lewis again appeals to our logic to guide us to a conclusion showing that the methods by which the devils produce their desired effect are numerous and varied; Lewis, through Screwtape, gives an example of just how varied they can be, "All extremes ... are to be encouraged. Not always, of course, but at this period. Some ages are lukewarm and complacent, and then it is our business to soothe them yet faster asleep," (33; ch.7) the methods given are opposing, so Lewis builds the logical basis for the belief that the devils will do it any way that works. Throughout the work Lewis gives many examples of possible methods of the devils such as, "tortured fear and stupid confidence are both desirable states of mind," (67; ch.15) the devils would encourage either state of mind. So the logical conclusion is that we will be face a wide array of methods of temptation depending on the age in which we live, and the temptations to which we are most susceptible.
Lewis presents another method of the devils as Screwtape warns Wormwood of "strengthening in [his] patient the fatal habit of attending to universal issues," (8; ch.1) and that he should not argue about "doctrines as primarily `true' or `false,'" (8; ch.1) thus we may logically conclude that in order to overcome the temptations of the devils we must think upon the eternities, and consider what is true and what is false. Perhaps this is why Lewis writes in such a way as to require personal thought, so we will think of what is true and false and thwart the devils.
Lewis gives further insight into the methods of the devils through the words of Screwtape, "The safest way to Hell is the gradual one--the gentle slope, soft underfoot, without sudden turnings, without milestones, without signposts,"(56;ch.12) So we know that the devils are not going to be overt. "Murder is no better than cards if cards can do the trick," (56; ch.12) Lewis, through the words of Screwtape, explains just how small an effective attack can be. Lewis's use of cards as a sufficient method shows just how subtle the way to Hell can be, as many would argue that the use of cards is not wrong at all, but it is what it leads to. If cards are all it takes to get someone to put God out of his mind, to disregard God's word, then it is sufficient, and the devils know it. So again he leads us to a logical conclusion: to protect ourselves from devils we must be aware of the small things that separate us from God.
Through all of this Lewis makes no conclusions. He leaves that to us. As we consider such things as, "So do not allow any temporary excitement to distract you from the real business of undermining faith and preventing the formation of virtues," (25; ch.5) we may learn more of how we should act, perhaps that we should do the opposite, but Lewis never says it. By leaving the conclusions to us Lewis helps us to form our own personal methods of protection, suited to our own strengths and weaknesses.
In his book, The Screwtape Letters, CS Lewis successfully instructs us how to protect ourselves from devils. He carefully appeals to our emotions through his use of diction and characterization of the devils to convince us of a need to protect ourselves. He then gives us logical instruction, but he leaves the conclusions to us, allowing us to form our own methods of protection.