תנה בני לבך לי ועיניך דרכי תצורנה, "Give your heart to me my son, and let your eyes observe my ways.⁠" (Proverbs 23,26)

In this verse Solomon warns that we should devote our hearts to the service of the Lord, and distance ourselves from the allurements of this world, the ones to which the heart and eyes are so attracted. This is why he says: "give your heart to Me:, i.e. to the part of the world which concerns service to your Creator and not to the alternative, the transient pleasures of terrestrial earth. Seeing that we do possess these urges, we are asked to channel them into permissible paths, i.e. service of the Lord. When we do this (wholeheartedly, not merely 'giving to religion its due') then our service to the Lord will be received with goodwill, seeing it reflects true reverence and love for God. Use of the pleasures of this world should be designed only to better serve the Lord and to better understand His Torah, at least the מושכלות, the laws accessible to our intelligence. Since many of the pleasurable experiences in this world are directly connected to the performance of commandments, what matters is the intent which governs the use one makes of these pleasures.
David summed up his own lifestyle and what prompted it in these words: ה', נגדך כל תאותי, "Lord, all my (natural) desires are focused in Your direction.⁠" He complimented himself that he had been able to sublimate the enjoyment inherent in partaking of the pleasures of this world so that they became religious experiences for him. It is a well known fact that the various organs, limbs, in our body are agents of the heart and the eyes. They merely carry out what the eyes and heart desire. Of the two, the heart is the primary element. If one's heart is whole with Hashem, everything is wholehearted. If the heart is not wholly devoted to God everything one does is blemished. This is the meaning of רחמנא לבא בעי, 'God requires one's heart' (Sanhedrin 106).
A Midrashic approach understands the verse in Proverbs as an admonition by God to the people of Israel. God says to Israel: "if you will give me your heart and your eyes, I will know that you are truly Mine. If not, you are not Mine.⁠" The essence of all this is that the heart and the eyes are the true root which determines either our rapprochement to God or vice versa. They are the "brokers" of the sins We know this from Jerusalem Talmud Berachot 1,8: "proof that the heart and eyes are the "brokers" of sin is what the Torah writes in Numbers 15,39: "do not explore after your hearts and after your eyes which you tend to stray after.⁠" The expression זונים, "straying,⁠" is the same as זונה, a harlot, a woman who strays after men not her husband. By using this term the Torah makes it plain that whoever is motivated in his actions by the dominance of his heart or his eyes is equated with a harlot. He allows himself to be seduced by his evil urge, as does a harlot. When Solomon follows up in Proverbs 23,27 with "for a harlot is a deep pit, a forbidden (alien) woman is a narrow well,⁠" he suggests that he who approaches the entrance of the house of a harlot will not escape the trap and damage which harlotry entails, just as someone who falls into a narrow well will not escape some damage. The same happens to anyone who gives in to the dictates of his hearts and eyes.
The simile of the alien woman being compared to a narrow deep well means that even if the alien woman is not engaged in promiscuous sexual activity, the fact is that if someone has his eyes on that well (woman) all the time and draws water from it, it is in the nature of things that the water he draws will be replaced by other water, perpetuating the inquisitiveness of the person confronted with the narrow well.
Solomon defines the allure of a woman as initially the making of conversation, שיחה, i.e. spelling the word שיחה (with the dot on the right side of the letter ש with a sin instead). Having engaged in conversation with such an alien woman, the next step is to look at her, followed by desiring her in one's fantasies; this will eventually result in one's touching such a woman, and the next step is the reversal of the sin again to a shin, i.e. שיחה, meaning "pit,⁠" the pit of sin. Solomon wrote these two verses sequentially in order to drive home the point to the people that they should devote their hearts and eyes to the spiritual part of life, to God and not to the material part of life characterized by our cravings. It is these cravings, or rather our giving in to them, which result in people forfeiting both their life on earth and their hereafter. The brief enjoyment resulting from indulging one's cravings is not worth what we have to give up in exchange.
The Torah goes to great length in legislating against anything which could result in harlotry as the sexual promiscuity and perverseness of the Canaanites was proverbial. Even the army of the Jewish people had to maintain standards of sanctity in war, so much so that the Torah did not permit the converting of prisoners of war to Judaism (for the purpose of marriage) until after a whole string of special conditions had been met.

כי תצא למלחמה על אויביך ונתנו ה' אלו-היך בידך ושבית שביו, "When you go into battle against your enemies and the Lord your God delivers him into your hand and you take as (its) prisoner.⁠" We would have expected the Torah to write ונתנם, "and He will deliver them into your hand,⁠" instead of "and He will deliver him into your hands.⁠" The Torah means that the defeat of your enemies will be so drastic as if they had all been only one man. Alternatively, the word ונתנו refers to the protective angel of the enemy nation in the celestial regions. The words ושבית שביו refer to the enemies here on earth.