Express and implied authority: agent powers can be inferred from habitual conduct enabling commercial acts on the principal's behalf. Section 187 distinguishes express authority given by words, spoken or written, from implied authority inferred from the circumstances, including things spoken or written and the ordinary course of dealing; habitual conduct-such as a shop manager ordering goods and paying from the principal's funds with the principal's knowledge-constitutes implied authority to act in the principal's name for the purposes of the shop.