Even though the subject and object are one and the same, the object is still present in the sentence, so you must use lay. The past tense of lay is laid, but be careful with the past tense of lie. Are you in the habit of laying down the law to everyone who will permit it. And laying down the magazine, he takes up her hand and presses it to his lips. I lay down my pen here, that you may consider of it a little, if you please.