Belief in the unseen (al-Ghayb) is a foundational principle of 'Aqeedah (creed) and a defining characteristic of a Muslim. Allah the Exalted praises the believers at the very beginning of the Qur'an by highlighting this quality: "[This is the Book in which there is no doubt, a guidance for the righteous]... Who believe in the unseen, establish prayer, and spend out of what We have provided for them" (Al-Baqarah 2:2-3).

Shaykh Ibn 'Uthaymeen, in his commentaries on creed like 'Sharh al-Aqeedah al-Wasitiyyah', explains that the 'unseen' encompasses all matters that are beyond the reach of our five senses, but about which we have been informed by Allah and His Messenger (peace be upon him). The reason Muslims believe in these matters is not because of philosophical proof or material evidence, but out of complete faith and trust in the divine revelation. To reject them is to reject the word of Allah and His Messenger.

This belief includes:

1. Angels: Beings created from light who are obedient servants of Allah. We believe in their existence, names (like Jibril and Mika'il), and the tasks assigned to them, such as recording deeds and taking souls at death, because the Qur'an and Sunnah explicitly mention them.
2. Jinn: Beings created from a smokeless flame of fire. Unlike angels, they possess free will, and among them are believers and disbelievers (shayateen/devils). Their existence is a reality confirmed by the texts.
3. Other Unseen Matters: This also includes belief in Allah's attributes, the events of the grave, the Resurrection, the Day of Judgment, Paradise, and Hellfire.

The Muslim's obligation is to affirm what is mentioned in the authentic texts without questioning 'how' and without trying to comprehend their ultimate nature beyond what is revealed. It is a major innovation (bid'ah) and misguidance to delve into the unseen using one's intellect, speculation, or worldly sciences, or to deny any part of it because it seems illogical to one's limited mind. The correct approach is complete submission to the revelation.