Published 14 Feb 2016
These are the core principles that is part of the Lightning Design System that directs the design of the Salesforce ecosystem.
I encourage you to check out the Lightning Design System site as well as the article Defining Principles to Drive Design Decisions by the Design Arcitect at Salesforce, JD Vogt, which describes how these principles are used in order of priority to drive design decisions.
Source: The Lightning Design System
Eliminate ambiguity. Enable people to see, understand, and act with confidence.
Streamline and optimize workflows. Intelligently anticipate needs to help people work better, smarter, and faster.
Create familiarity and strengthen intuition by applying the same solution to the same problem.
Demonstrate respect for people’s time and attention through thoughtful and elegant craftsmanship.