Tiantai, also known as the Tendai school in Japan, is a Mahayana Buddhist tradition that originated in China during the 6th century. It is based on the teachings of the Lotus Sutra and is known for its emphasis on integrating all Buddhist teachings into one comprehensive practice.

The core principles of Tiantai can be summarized as follows:

1. The Middle Way: Tiantai teaches the Middle Way, which is the path between extremes. It emphasizes the importance of avoiding attachment to either extreme and instead finding balance and harmony in all aspects of life.

2. The Threefold Truth: This principle states that all phenomena are ultimately empty, but also have a temporary existence and a conventional reality. Tiantai teaches that these three truths are interdependent and must be understood together to gain a complete understanding of reality.

3. The Five Periods and Eight Teachings: Tiantai divides the Buddha's teachings into five periods and eight teachings. The five periods represent the different stages of the Buddha's teachings, and the eight teachings represent the different methods and practices for attaining enlightenment.

4. The Ten Worlds: Tiantai teaches that all beings possess the potential for enlightenment and can exist in ten different states of life, known as the Ten Worlds. These include the worlds of hell, hunger, animality, anger, humanity, heaven, learning, realization, bodhisattva, and Buddhahood.

5. The One Vehicle: Tiantai emphasizes the universal potential for enlightenment and teaches that all beings can attain Buddhahood through the practice of the Lotus Sutra. This is known as the One Vehicle, which encompasses all other vehicles and teachings.

6. The Threefold Lotus Sutra: Tiantai focuses on the Lotus Sutra as the ultimate teaching of the Buddha. It is seen as the most profound and comprehensive of all the sutras, containing the essence of all other teachings.

7. The Threefold Contemplation: This principle involves the practice of meditation and contemplation on the threefold truth, the ten worlds, and the threefold Lotus Sutra. Through this practice, one can gain insight into the true nature of reality and achieve enlightenment.

In conclusion, the core principles of Tiantai revolve around the Middle Way, the threefold truth, the five periods and eight teachings, the ten worlds, the One Vehicle, the threefold Lotus Sutra, and the threefold contemplation. These principles emphasize the universal potential for enlightenment and the integration of all Buddhist teachings into one comprehensive practice.