That's strange, because 8,000 natural men heard and believed the Gospel on the day of Pentecost, Acts 2:41 also Acts 4:4.
Yep.
And Paul was referring to various, failed means of relying on the senses for perception.
His meaning is that his spirit had received information about the things of God from the Spirit of God Himself (which is why the lower case "spirit" in those passages, at least in the KJV).
Paul was contrasting what had been the basis of his perception with what is the basis of other modes of perception.
The thing there about the natural man is the issue of how that what Paul's "spirit" had received from God's Spirit was not information that relying on one's natural senses is able to come up with, let alone, fathom.
He is not saying the natural man can not understand those things (after they are revealed), rather; that the natural man is not able to come up with them on his own - they were - the things of God - that only God had known of until He - revealed them by His Spirit - to - and through - Paul's spirit, to his readers.
Readers who though saved, could not discern his words on said things of God because they were "yet carnal" 1 Cor. 3.
This is all the same issue Paul has been going on about in that chapter, in different words, each time.
The sense of it all is the same as that of Peter's in 2 Peter 1.
2 Peter 1:19 We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts: 1:20 Knowing this first, that no prophecy of the scripture is of any private interpretation. 1:21 For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.
Both men were merely talking about The Process of Devine Inspiration.
Peter's having been in connection with that which had been Prophesied.
In contrast, Paul's sense had been in connection with that which had not been Prophesied...
1 Corinthians 2:9 But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.
Note the contrast between those Believers whom Paul relates Isaiah had written that to, and those Believers to whom Paul writes about the following revelation to, and he then goes into, for a brief moment....
1 Corinthians 2:10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
Acts 17:11, 12.