The English language boasts a wide range of synonyms for the phrase "without thinking." Some of the most commonly used alternatives include "mindlessly," "automatically," "unconsciously," and "instinctively." Other possible substitutions include "absentmindedly," "unwittingly," "recklessly," and "impulsively." Each of these synonyms conveys a slightly different nuance or implication, depending on the specific context in which it's used. "Mindlessly" might suggest a lack of focus or awareness, while "instinctively" might indicate a more reflexive response. Ultimately, the choice of synonym will depend on the speaker or writer's intended meaning and tone.