Tendai, also known as Japanese Tiantai, is a Buddhist school of thought that emerged in Japan in the 9th century. It was founded by the monk Saicho, who studied and synthesized the teachings of Chinese Tiantai Buddhism with Japanese Buddhism. Tendai philosophy is based on the Lotus Sutra, which is considered the highest and most comprehensive teaching of the Buddha.

The key principles of Tendai philosophy can be summarized as follows:

1. The Middle Way: Tendai emphasizes the importance of finding balance and avoiding extremes in one's spiritual practice. This concept is based on the Buddhist teaching of the Middle Way, which advocates for avoiding both indulgence and asceticism.

2. The Three Truths: Tendai teaches that there are three levels of truth: conventional truth, provisional truth, and ultimate truth. Conventional truth refers to the everyday reality we experience, provisional truth refers to the teachings and practices that help us reach enlightenment, and ultimate truth refers to the ultimate nature of reality, which is beyond concepts and words.

3. The Threefold Truth: This principle states that all phenomena are empty of inherent existence, yet they manifest in various forms and have the potential to lead to enlightenment. It emphasizes the interconnectedness and interdependence of all things.

4. The Threefold Lotus Sutra: The Lotus Sutra is considered the most important text in Tendai philosophy. It is divided into three parts: the theoretical teaching, the parables, and the ultimate teaching. Each part represents a different aspect of the Buddha's teachings and is meant to be understood and practiced together.

5. The Fourfold Practice: Tendai emphasizes the importance of incorporating four types of practice in one's spiritual journey: meditation, study, ritual, and compassionate action. These practices help cultivate wisdom, compassion, and ultimately lead to enlightenment.

6. The Fourfold Truth: This principle states that all beings possess the Buddha-nature, and therefore have the potential to attain enlightenment. It also emphasizes the cyclical nature of existence, with beings constantly being born and reborn in different forms.

7. The Six Paramitas: Tendai teaches the six paramitas or perfections, which are generosity, morality, patience, effort, concentration, and wisdom. These virtues are seen as essential for spiritual development and are meant to be cultivated in daily life.

In conclusion, Tendai philosophy is a synthesis of various Buddhist teachings, emphasizing the Middle Way, the Three Truths, the Threefold Lotus Sutra, and the importance of balanced practice. It also emphasizes the potential for all beings to attain enlightenment and the cultivation of virtues such as compassion and wisdom. These principles continue to guide and inspire practitioners of Tendai Buddhism to this day.