You should look at the context of the rest of the Surah.
And when our verses are recited to them as clear evidences, they say, "This is not but a man who wishes to avert you from that which your fathers were worshipping." And they say, "This is not except a lie invented." And those who disbelieve say of the truth when it has come to them, "This is not but obvious magic." [34:43]
And those before them denied, and the people of Makkah have not attained a tenth of what We had given them. But the former peoples denied My messengers, so how [terrible] was My reproach. [34:45]
Say, "Whatever payment I might have asked of you - it is yours. My payment is only from Allah , and He is, over all things, Witness." [34:47]
In context, the rest of the Surah is about how people used to deny the prophets that Allah has sent to them. The part about "there is no madness in your companion" is to say that the prophet (your companion) is not mad, and is only preaching Allah's word.
give thought in this context means to think carefully of what happened to those who reviled the previous prophets, how they were treated, and the punishments that was sent to them. This was likely directed towards disbelievers of the time of the prophet Muhammad (pbuh) who doubted whether he was really a Prophet or not.